If you go a length 'l' in the -x direction of a linear function y=kx+m from a point (a,b), what's the coordinates of the point you end up in?

So you are given    b = ka +m     then move -l

                                     k(a-l) +m

                                      ka-kl  +m

                                      (ka + m) -kl        remember we were givien ka+m = b  (see above)

                                       b - kl        equals the new 'y'

the new points would be

x,y =  ( (a-l) , (b-kl) )