Question: what is the formula to find the other endpoint?
Answer: The formula to use is a derivation of the better-known midpoint formula. Letting (m1, m2) represent the given midpoint, (x1, y1) represent the given endpoint, and (x2, y2) represent the unknown endpoint, the formula is: (x2, y2) = (2_m1 – x1, 2_m2 – y1).