Question

Find the product of the matrices, if exists. $(\begin{array}{cc}2& -1\end{array})(\begin{array}{c}5\\ 4\end{array})$

Answer 1

${\left[\begin{array}{cc}2& -1\end{array}\right]}_{1\times 2}{\left[\begin{array}{c}5\\ 4\end{array}\right]}_{2\times 1}$

$=[2\times 5-1\times 4]$

$=\left[6\right].$

Hence, the answer is $6.$