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