M\left(\begin{array}{c} 1 \\ 2\end{array}\right) = M^t \left(\begin{array}{c} 2 \\ 1\end{array}\right) \Leftrightarrow \left(\begin{array}{c} m_1 + 2m_2 \\ m_3 + 2m_4\end{array}\right) = \left(\begin{array}{c} 2m_1 + m_3 \\ 2m_2 + m_4\end{array}\right) \Leftrightarrow \\ \\ m_1 + 2m_2 = 2m_1 + m_3 \\ m_3 + 2m_4 = 2m_2 + m_4