[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points }\\\\
\begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&C&(~ -9 &,& -4~)
% (c,d)
&D&(~ x &,& y~)
\end{array}\qquad
% coordinates of midpoint
\left(\cfrac{ x_2 + x_1}{2}\quad ,\quad \cfrac{ y_2 + y_1}{2} \right)
\\\\\\
\left( \cfrac{x-9}{2}~~,~~\cfrac{y-4}{2} \right)=\stackrel{midpoint~M}{(-1~~,~~-2)}\implies
\begin{cases}
\cfrac{x-9}{2}=-1\\\\
x-9=-2\\
\boxed{x=7}\\
----------\\
\cfrac{y-4}{2}=-2\\\\
y-4=-4\\
\boxed{y=0}
\end{cases}[/tex]
