[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points }
\\\\
\begin{array}{ccccccccc}
&&x_1&&y_1&&x_2&&y_2\\
% (a,b)
&&(~ 4 &,& 2~)
% (c,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+4}{2}~~,~~\cfrac{y+2}{2} \right)=\stackrel{midpoint}{(-2~~,~~-1)}\implies
\begin{cases}
\cfrac{x+4}{2}=-2\\\\
x+4=-4\\
\boxed{x=-8}\\
----------\\
\cfrac{y+2}{2}=-1\\\\
y+2=-2\\
\boxed{y=-4}
\end{cases}[/tex]
