text: You find midpoint by adding the 2 x-coordinates of the endpoints and dividing by 2, and doing the same for the y-coordinates.  So if you know the midpoint and one end, it looks like this (assume the other endpoint is (x,y):\n\n(4+x)/2 = 10; (-4+y)/2 = 12\n\nMultiply by 2:\n4+x=20; -4+y=24\n\nSo x=16 and y=28