Chapter 10 - 10.2 - Operations with Matrices - 10.2 Exercises - Page 725: 56

$\begin{bmatrix} 12 & 6\\ -4 & -2\\ 20 & 10\\ 28 & 14\\ \end{bmatrix}$

First add the matrices in the parentheses: $\begin{bmatrix} 5+7-8 & -6-1+9\\ \end{bmatrix}$ = $\begin{bmatrix} 4 & 2\\ \end{bmatrix}$ Then multiply the matrix outside of the parentheses with the one found above: $\begin{bmatrix} 3\\ -1\\ 5\\ 7\\ \end{bmatrix}$$\begin{bmatrix} 4 & 2\\ \end{bmatrix}$ = $\begin{bmatrix} 3(4) & 3(2)\\ -1(4) & -1(2)\\ 5(4) & 5(2)\\ 7(4) & 7(2)\\ \end{bmatrix}$ = $\begin{bmatrix} 12 & 6\\ -4 & -2\\ 20 & 10\\ 28 & 14\\ \end{bmatrix}$