The midpoint of a segment is (2,-5), and one of the endpoints is (3,6). where is the other endpoint?

(2, -5) is the mid point

(3, 6) is (2+1, -5+11) is an end point

therefor the other end point should be (2-1, -5-11) = (1, -16)

You could visualise this better if you draw it

Hope this helps, I don't really know a better way to explain it, sorry =P