draw_line.ss

(define image (line 0 0 'white))
(define (draw-line v1 v2)
  (set! image 
        (add-line image
                  (xcor-vect v1) ; x-coordinate ...etc
                  (ycor-vect v1)
                  (xcor-vect v2)
                  (ycor-vect v2)
                  'black)))