How do you find the missing endpoint of a segment with an endpoint and a midpoint?

The direction of missing endpoint is the same as the direction from the known end point to the midpoint.

The distance from the midpoint to the missing endpoint is the same as the distance from the known end point to the midpoint.

In coordinate geometry it is simple.

If the known end point is (p, q) and the mid point is (r, s) then the missing point is (2r - p, 2s - q).