#### Answer

28

#### Work Step by Step

The determinant of a $2 \times 2$ matrix can be computed by using the formula $det =ps-qr$
where $det =\begin{bmatrix}p & q \\r & s\end{bmatrix}$
We have $det =\begin{bmatrix} 7 & 0 \\0 & 4\end{bmatrix}$
or, $det =(7)(4)-(0)(0)=28$