Great question! A midpoint is exactly what it sounds like; the point in the middle of two objects (in this case it's the middle of these two coordinates). Another way to think of the middle of something in math is by using the average. To find the midpoint of two points, we just find the average of the x-values and the average of the y-values:
((x1 + x2)/2 , (y1 + y2)/2)
((5 + -13) / 2 , (-4 + 12) / 2)
( -8 / 2 , 8 / 2)
(-4 , 4)