[tex]\bf \textit{middle point of 2 points }\\ \quad \\
\begin{array}{lllll}
&x_1&y_1&x_2&y_2\\
% (a,b)
R&({{ x}}\quad ,&{{ y}})\quad
% (c,d)
S&({{ 8}}\quad ,&{{ 9}})
\end{array}\qquad
% coordinates of midpoint
\left(\cfrac{{{ x_2}} + {{ x_1}}}{2}\quad ,\quad \cfrac{{{ y_2}} + {{ y_1}}}{2} \right)[/tex]
[tex]\bf \left( \cfrac{8+x}{2}~~,~~\cfrac{9+y}{2} \right)=\stackrel{midpoint}{M(6,6)}\implies
\begin{cases}
\cfrac{8+x}{2}=6\\\\
8+x=12\\
\boxed{x=4}\\
----------\\
\cfrac{9+y}{2}=6\\\\
9+y=12\\
\boxed{y=3}
\end{cases}[/tex]
