The coordinates of the midpoint of two points A and B are the averages of the corresponding coordinates of A and B.
So if one point is (a, b) and another point is (c, d), the point exactly between them has coordinates ((a+b)/2, (c+d)/2).
In this example, the points
A(-3a, b)
B(3a, b)
have midpoint
F((-3a+3a)/2, (b+b)/2) = F(0, 2b/2) = F(0, b)
