Matrix elements

$A=\left[\begin{array}{rr}21 & -12 & 18 & 6 \\-8 & 3 & 4 & -1 \\2 &2 &-8 & 4 \\1 &9 &-15 & -24 \\2 &2 &1 & 4\end{array}\right]$ **$A_{5,1}=$** [[☃ numeric-input 1]]