Writing flatten method in Scheme

Question

I have been working on the following function flatten and so far have it working for just lists. I was wondering if someone could provide me with some insight on how to get

Answer 1

Here's one option:

(define (flatten x)
  (cond ((null? x) '())
        ((pair? x) (append (flatten (car x)) (flatten (cdr x))))
        (else (list x))))