To find midpoint between points $ A(x_1,y_1) $ and $ B(x_2,y_2) $, we use formula:

$$ M = \left( \frac{x_1 + x_2}{2}, \frac{y_1 + y_2}{2} \right) $$

In this example we have:

$$ \begin{aligned} & A \left(-3,~9\right) \implies x_1 = -3 ~~\text{and}~~ y_1 = 9 \\[1 em] & B \left(3,~9\right) \implies x_2 = 3 ~~\text{and}~~ y_2 = 9 \end{aligned} $$

After substituting into above formula, we get:

$$ \begin{aligned} M & \left( \frac{ -3 + 3 }{2}, \frac{ 9 + 9 }{2} \right) \\[1 em] M & \left( \frac{ 0 }{2}, \frac{ 18 }{2} \right) \\[1 em] M & \left(0,~9\right) \end{aligned} $$

Midpoint Calculator