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