(This is homework, however I have a working solution that I'm trying to improve upon)

I'm working on a program that adds or subtracts up to 15 decimal numbers from the command line. I have a working program, but I am trying to optimize it to reduce some redundancy. Basically my add and subtract functions are identical other than some sign switches, and I feel like there might be a better way. Here is what I have so far (the program can accept decimal input, or hex input formatted with a preceding 0x, which is why there are a sections dealing with those conversions):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

void menu(char **options){
  for(int i = 0; i < 3; ++i){
    printf("%d. %s\n", i, options[i]);
  }
  printf("Menu Item: ");
}

int isHex(char *argv, int len){
  
  if(argv[0] == '0' && (argv[1] == 'x' || argv[1] == 'X')){
    for (int i = 2; i < len; ++i)
      if(!isxdigit(argv[i]))
        return 0;
    return 1;
  }

  return 0;
}

int convertHex(char *hexVal, int len){

  int dec_val = 0;
  int base = 1;

  for (int i=len-1; i>=2; i--) {    
    if (hexVal[i]>='0' && hexVal[i]<='9'){ 
        dec_val += (hexVal[i] - 48)*base; 
        base = base * 16; 
    } 

    else if (hexVal[i]>='A' && hexVal[i]<='F') { 
        dec_val += (hexVal[i] - 55)*base;
        base = base*16; 
    } 
  }
  
  return dec_val; 
}

void add(int argc, char **argv){
  int i = 1, s = 0, x = 0;

  if(isHex(argv[i], strlen(argv[i])))
    s = (x =convertHex(argv[i], strlen(argv[i])));

  else
    s = atoi(argv[i]); 

  printf("%s + ", argv[i]);

  for(i = 2;i<argc;i++){
    if(isHex(argv[i], strlen(argv[i]))){
      s += (x = convertHex(argv[i], strlen(argv[i])));
      printf("%s", argv[i]);
    }

    else{
      printf("%s", argv[i]);
      s += atoi(argv[i]);
    }

    if(i == argc-1)
      printf(" = ");
    else
      printf(" + ");
   }

   printf("%d\n", s);
}

void sub(int argc, char **argv){

  int i = 1, s = 0, x = 0;

  if(isHex(argv[i], strlen(argv[i])))
    s = (x =convertHex(argv[i], strlen(argv[i])));

  else
    s = atoi(argv[i]); 

  printf("%s - ", argv[i]);

  for(i = 2;i<argc;i++){
    if(isHex(argv[i], strlen(argv[i]))){
      s -= (x = convertHex(argv[i], strlen(argv[i])));
      printf("%s", argv[i]);
    }

    else{
      printf("%s", argv[i]);
      s -= atoi(argv[i]);
    }

    if(i == argc-1)
      printf(" = ");
    else
      printf(" - ");
   }

   printf("%d\n", s);
}

int main(int argc,char** argv){

  char *choices[] = {"Exit", "Addition", "Subtraction"};
  void (*calc[])(int, char**) = {NULL, add, sub};


  if(argc < 3)
    fprintf(stderr, "You must enter at least two arguments.\n");

  else if(argc > 16)
    fprintf(stderr, "You cannot enter more than 15 arguments.\n");

  else{

    int ch=1;
    while(ch!=0){   
      menu(choices);
      scanf("%d",&ch);

      switch(ch){
        case 0:
          exit(0);
          break;

        case 1:
          calc[1](argc,argv);
          break;
        
        case 2:
          calc[2](argc,argv);
          break;
          
        default:
          printf("Enter valid option\n");
          break;
      }
    }

    exit(EXIT_SUCCESS);
  }

  exit(EXIT_FAILURE);
}

The relevant functions are add/sub. It gets tricky because of needing to output the operations to the user. My initial inclination was to go with something like an op-code 1 = add, 2 = sub, but I don't know if that just adds more if-checks in one function and ultimately makes it as long or harder to read.

Example:

void calc(int argc, char **argv, int op){

  int i = 1, s = 0, x = 0;
  char sign = (op == 1) ? '+' :  '-';

  if(isHex(argv[i], strlen(argv[i])))
    s = (x =convertHex(argv[i], strlen(argv[i])));

  else
    s = atoi(argv[i]); 

  for(i = 2;i<argc;i++){

    //Add
    if(op == 1){
      if(isHex(argv[i], strlen(argv[i]))){
        s += (x = convertHex(argv[i], strlen(argv[i])));
        printf("%s", argv[i]);
      }

      else{
        printf("%s", argv[i]);
        s += atoi(argv[i]);
      }

      if(i == argc-1)
        printf(" %c ", sign);
      else
        printf(" %c ", sign);
    }

    //Repeat for subtract

    //Print total
    printf("%d\n", s);
  }
}

But that seems like just making 1 function larger without really optimizing anything.

Does anyone have any suggestions or helpful advice for reducing the redundancy with add/sub (if its even possible)? Its not entirely necessary, but I'd appreciate any assistance.