You've already got an inventory of commented expressions you're likely to need in solving the problem. In this step, add an (un-commented) expression built from the inventory and whose value will produce the correct answer to the function.
For example, in the cube function, the header and inventory give us
(define (cube side) ; side a number )This becomes
(define (cube side) ; side a number (* side side side) )because the cube of a number is computed by multiplying together three copies of that number.