From the Extended Exercise beginning in section 2.1.4, you can find exercise 2.8:
Exercise 2.8. Using reasoning analogous to Alyssa's, describe how the difference of two intervals may be computed. Define a corresponding subtraction procedure, called sub-interval.
I wrote the following:
(define (sub-interval x y) (make-interval (- (lower-bound x) (upper-bound y)) (- (upper-bound x) (lower-bound y))))
What do you think?