[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points } \\\\ A(\stackrel{x_1}{1}~,~\stackrel{y_1}{-1})\qquad B(\stackrel{x_2}{x}~,~\stackrel{y_2}{y}) \qquad \left(\cfrac{ x_2 + x_1}{2}~~~ ,~~~ \cfrac{ y_2 + y_1}{2} \right) \\\\\\ \left( \cfrac{x+1}{2}~~,~~\cfrac{y-1}{2} \right)~~=~~\stackrel{M}{(3~~,~~1)}\implies \begin{cases} \cfrac{x+1}{2}=3\\[1em] x+1=6\\ \boxed{x=5}\\[-0.5em] \hrulefill\\ \cfrac{y-1}{2}=1\\[1em] y-1=2\\ \boxed{y=3} \end{cases}[/tex]
