If $A = \left[\begin{array}{cc} -4 &1\cr -4 &4 \end{array}\right]$ and $B = \left[\begin{array}{cc} 6 &-4\cr 3 &-1 \end{array}\right]$, then the commutator of $A$ and $B$ is

$\lbrack A, B \rbrack =$ $\left[\Rule{0pt}{2.4em}{0pt}\right.$$\left]\Rule{0pt}{2.4em}{0pt}\right.$.