The coordinates of the midpoint of two other points is the average of their coordinates.
So let the coordinate of the missing endpoint be (x,y).
Then: the average x coordinate is (x+19)/2=18 and the average y coordinate is (y-15)/2=-18.
From these equations: x+19=36, so x=36-19=17; y-15=-36, y=15-36=-21.
Therefore the coordinates of the endpoint is (17,-21).