The formula of a midpoint of AB:
[tex]M_{AB}\left(\dfrac{x_A+x_B}{2};\ \dfrac{y_A+y_B}{2}\right)[/tex]
We have:
G(b; 0); F(3b; 2b)
substitute:
[tex]M_{GF}\left(\dfrac{b+3b}{2};\ \dfrac{0+2b}{2}\right)\to M_{GF}(2b;\ b)[/tex]
Answer: (2b, b)
