Chapter 4, Problem 9RE

In Problem $6\text{-}14$, perform the operations that are defined, given the following matrices:

$A=\left[\begin{array}{cc}1& 2\\ 3& 1\end{array}\right]B=\left[\begin{array}{cc}2& 1\\ 1& 1\end{array}\right]C=\left[\begin{array}{cc}2& 3\end{array}\right]D=\left[\begin{array}{c}1\\ 2\end{array}\right]$

$AB$