Answer
$\pm 4$
Work Step by Step
The determinant of a $2 \times 2$ matrix can be computed by using the formula $det =ps-qr$
where $det =\begin{bmatrix}p & q \\
r & s\end{bmatrix}$
After simplifying, this matrix yields: $2 0=(x)(x) - (4)(-1) $
or, $x^2+4=20 \implies x=\pm 4$