[tex]\bf ~~~~~~~~~~~~\textit{middle point of 2 points } \\\\ R(\stackrel{x_1}{x}~,~\stackrel{y_1}{y})\qquad S(\stackrel{x_2}{3}~,~\stackrel{y_2}{9}) \qquad \left(\cfrac{ x_2 + x_1}{2}~~~ ,~~~ \cfrac{ y_2 + y_1}{2} \right) \\\\\\ \left( \cfrac{3+x}{2}~~,~~\cfrac{9+y}{2} \right)~~=~~\stackrel{M}{(2~,~5)}\implies \begin{cases} \cfrac{3+x}{2}=2\\[1em] 3+x=4\\ \boxed{x = 1}\\ \cline{1-1} \cfrac{9+y}{2}=5\\[1em] 9+y=10\\ \boxed{y=1} \end{cases}[/tex]
