Explanation:
in essence, these 2 points are the Hypotenuse of a right-angled triangle. the legs are the coordinate differences between the 2 points creating the 3rd point of that triangle at (-10, 7) : the x coordinate of one point, and the y coordinate of the other.
now, we are looking for the midpoint. this midpoint cuts the original line/hypotenuse in half creating a scaling factor of 1/2.
when we create now a similar triangle based on that "half- Hypotenuse", we need to use the same scaling factor also on the legs (the coordinate differences).
so, when we cut the coordinate differences also in half, we get the coordinates for the midpoint on the main line.
xm, ym are the coordinates of the midpoint.
xm = (x1 + x2)/2 = (-10-3)/2 = -13/2 = -6.5
ym = (y1 + y2)/2 = (-6+7)/2 = 1/2 = 0.5
so, the midpoint is (-6.5, 0.5)