(define a '(1 3 (5 7) 9))
(define a7 (car (cdr (car (cdr (cdr a))))))

(define b '((7)))
(define b7 (car (car b)))

(define c '(1 (2 (3 (4 (5 (6 7)))))))
(define c7 (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr (car (cdr c)))))))))))))