#linear-transform

$\mathbf{x}$ is uniformly distributed on the unit square $[0, 1]^2$ (density 1 there), and $\mathbf{y} = A\mathbf{x}$ with $A = \begin{pmatrix} 2 & 1 \\ 1 & 3 \end{pmatrix}$. What is the density $p_{\mathbf{y}}(\mathbf{y})$ at a point inside the image of the square?