m \left (\begin{array}{cc} 0 & c\\ 0 & y \end{array} \right) = \bigg(m \left (\begin{array}{cc} 0 & c\\ 0 & y \end{array} \right)\bigg)\varepsilon_0 = m\bigg( \left (\begin{array}{cc} 0 & c\\ 0 & y \end{array} \right)\varepsilon_0\bigg) =m\varepsilon_0=m