Matrix concept and types of matrices

1. $\left(\begin{array}{cccc} 2 & 1 & 5 & 0 \ -3 & 2 & -1 & 3 \end{array} \right)$

2. $\left(\begin{array}{cc} 2 & 2 \ 0 & 9 \ 0 & 4 \end{array} \right)$

What values do the elements $a_{23}$ and $a_{12}$ have in the matrix $\left(\begin{array}{ccc} 42 & -13 & 0 \ 31 & 12 & 8 \end{array} \right)$

The element $a_{23}$ corresponds to row $2$ and column $3$, it is therefore $8$.

The element $a_{12}$ corresponds to row $1$ and column $2$, it is therefore $-13$.

$a_{23}=8$ and $a_{12}=-13$.