Answer
$\left[\begin{array}{cc}
{50}&{-3}\\
{18}&{21}\end{array}\right]$
Work Step by Step
$ AC=\left[\begin{array}{cc}{28}&{-9}\\{4}&{23}\end{array}\right]\quad$(see sol. exercise $15$)
$ BC=\left[\begin{array}{cc}{22}&{6}\\{14}&{-2}\end{array}\right]\quad$(see sol. exercise $16$)
Both are 2 by 2 matrices, so their sum is defined.
$AC+BC=\left[\begin{array}{cc}
{28+22}&{-9+6}\\
{4+14}&{23+(-2)}\end{array}\right]=\left[\begin{array}{cc}
{50}&{-3}\\
{18}&{21}\end{array}\right]$
