Add up the x coordinates to get 1+(-1) = 0. Divide this sum in half to get 0/2 = 0. Therefore the x coordinate of the midpoint is 0.
Repeat for the y coordinates. First add the y coordinates to get -7+(-23) = -30. Then divide that result in half to get -30/2 = -15. So the y coordinate of the midpoint is -15.
