(define (foo xs)
  (cond ((null? xs) ()) ; Base case 1: The empty list
        ((null? (cdr xs)) (cons (* 2 (car xs)) '())) ; Base case 2: The singleton
        (else (cons (* 2 (car xs)) (cons (cadr xs) (foo (cddr xs))))))) ; Anything else

(display (foo '(3 4 5 6 7)))