Suppose we sample $(x_1,x_2)$ uniformly from unit circle, are interested in the
distribution of the following quantity.
$$
X=\left(\begin{array}{cc}x_1& x_2\end{array}\right)\cdot
\left(\begin{array}{cc}
3&0\\
0&1\\
\end{array}\right)\cdot
\left(
\begin{