How to find the missing endpoint when given the midpoint?

For y :

your are given one of the y coordinates, which we can call y1

and your midpoint for y is m(y)

so the missing y co-ordinate would be

(y1 - y)/2 = m(y)

then substitute in y1 and m.
then you can find the answer

do the same for x.

hope it helps. please tell if you need clarification.