First find the distance from the endpoint you the mid point. Because the endpoint and midpoint lie on the same x value but different y value you would add the absolute values of the two y values because they are in different quadrants. The distance from the end point and mid point is 11 units.
Because a midpoint halves two endpoints you would just add the distance to itself again to the midpoint. (5, 10+11) which will then give you (5, 21).