(x,y)-----(-20,-4)-----(-17,-5)
to go from -17 to -20 you subtract 3
to go from -20 to x you subtract 3 again to get -20-3=-23
to go from -5 to -4 you add 1
to go from -4 to y you add 1 again to get -4+1=-3
(-23,-3) (the midpoint is equidistant from both endpoints)
