[plt-scheme] setting variables

How do I set a variable used in a mathematical function using set!,
based upon what is put into a text-field (I'm using gui.ss)

example:
(define (Math x)
    (+ 1 a d)
 if user types "H" into text-box 1 than (set! a (+ a 1) and if user
types "O" into text-box 2 than (set! d (- d 2).