Chapter 4.2, Problem 12E

Problems $1\text{-}14$ refer to the following matrices: (If necessary, review the terminology at the beginning of Section 4.2.)

$\begin{array}{c}A=\left[\begin{array}{l}\begin{array}{ccc}2& -4& 0\end{array}\\ \begin{array}{ccc}6& 1& -5\end{array}\end{array}\right]\\ B=\left[\begin{array}{ccc}-1& 9& 0\\ -4& 8& 7\\ 2& 4& 0\end{array}\right]\\ C=\left[\begin{array}{ccc}2& -3& 0\end{array}\right]\\ D=\left[\begin{array}{c}-5\\ 8\end{array}\right]\end{array}$

For matrix $A$, list the elements $a_{21},a_{12}.$