#lang racket

(define (at-level lst n curdepth)
  (cond
   [(empty? lst) empty]
   [(list? (first lst)) (append
                         (at-level (first lst) n (+ 1 curdepth))
                         (at-level (rest lst) n curdepth))]
   [(= n curdepth) (cons (first lst) (at-level (rest lst) n curdepth))]
   [(at-level (rest lst) n curdepth)]))

(define a '(1 2 (3 4) 5 (6 7)))

(at-level a 2 1)