Answer
$-25$
Work Step by Step
The determinant $D$ of a $ 2 \times 2$ matrix can be calculated as:
$D=\left|\begin{array}{ll}
A & B\\C & D \end{array}\right|=AD-BC$
The determinant $D$ of a given $ 2 \times 2$ matrix can be calculated as:
$D=\left|\begin{array}{ll}
-1 & 8\\ 2 & 9 \end{array}\right|=(-1)(9)-(2)(8)=-25$
