$\left[\begin{array}{cc} -6 &1\cr 30 &-5 \end{array}\right]$ $\left[\Rule{0pt}{2.4em}{0pt}\right.$$\left]\Rule{0pt}{2.4em}{0pt}\right.$ $= \left[\begin{array}{cc} 0 &0\cr 0 &0 \end{array}\right]$.