If you are stuck on problem 6.11, hopefully this will help you get started...

(define (valid-date? month day year)
  (cond ((not (valid-month? month)) #f)
        ((31-day-month? month) (between? day 1 31))
        ((30-day-month? month) (between? day 1 30))
        ; If we get to the next line, must be February.  Why?
        ((leap-year? year) (between? day 1 29))
        (else (between? day 1 28))))