[math]\begin{array}{l}
A( - 1 + t; - 2 - t)\\
B(1 + t';2t')\\
\Rightarrow M\left( {\dfrac{{t + t'}}{2};\dfrac{{ - 2 - t + 2t'}}{2}} \right)\\
\Rightarrow \dfrac{{t + t'}}{2} = 1;\dfrac{{ - 2 - t + 2t'}}{2} = 1\\
\Leftrightarrow \left\{ \begin{array}{l}
t + t' = 2\\
t - 2t' = - 4
\end{array} \right.
\end{array}[/math]