To make things easier for us, let (-4 , -3) and (4, -1) be the line which forms the hypotenuse.
Thus, one of the possible third points, will have the x-value of (4,-1) and the y-value of (-4, -3).
This third point will be (4,-3)
Note that this is not the only possible third point. If you are having difficulty understanding, plot out three points here.