The formula of the coordinates of a midpoint:
[tex]M\left(\dfrac{x_1+x_2}{2};\ \dfrac{y_1+y_2}{2}\right)[/tex]
We have:
[tex]P(-9,\ -10)\to x_1=-9,\ y_1=-10\\\\Q(-9,\ 6)\to x_2=-9,\ y_2=6[/tex]
Substitute:
[tex]\dfrac{-9+(-9)}{2}=\dfrac{-18}{2}=-9\\\\\dfrac{-10+6}{2}=\dfrac{-4}{2}=-2[/tex]
Answer: [tex]M(-9,\ -2)[/tex]
