Chapter 3 Linear Systems and Matrices - 3.6 Multiplying Matrices - 3.6 Exercises - Skill Practice - Page 200: 29

$4AC+3AB=\begin{bmatrix} -204 & 81 \\ 160& -38 \end{bmatrix}$

We found in exercise 24 that: $AC=\begin{bmatrix} -42 & 12 \\ 28 & -2 \end{bmatrix}$ $AB=\begin{bmatrix} -12 & 11 \\ 16& -10 \end{bmatrix}$ $4AC+3AB=4\begin{bmatrix} -42 & 12 \\ 28 & -2 \end{bmatrix}+3\begin{bmatrix} -12 & 11 \\ 16& -10 \end{bmatrix}$ $=\begin{bmatrix} -168 & 48 \\ 112& -8 \end{bmatrix}+\begin{bmatrix} -36& 33\\ 48& -30 \end{bmatrix}$ $=\begin{bmatrix} -204 & 81 \\ 160& -38 \end{bmatrix}$