#### Answer

$3x^2+3y^2$

#### 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, the determinant of this matrix will be:
$det =(3)(x^2)-(-3)(y^2)$
or, $=3x^2+3y^2$