Add to the midpoint the difference between the midpoint and the end point. The result will be the other end point. Example end = (2, 20) mid = (6, 60) The difference "mid" - "end" is (6, 60) - (2, 20) = (4, 40). Add that to "mid" to get (6, 60) + (4, 40) = (10, 100) The other end point is (10, 100). We have concluded that (6, 60) is the midpoint between (2, 20) and (10, 100).