Answer
$-10$
Work Step by Step
We have $$
\operatorname{Jac}(G)=\frac{\partial(x, y)}{\partial(u, v)}=\left|\begin{array}{ll}
{\frac{\partial x}{\partial u}} & {\frac{\partial x}{\partial v}} \\
{\frac{\partial y}{\partial u}} & {\frac{\partial y}{\partial v}}
\end{array}\right|=\left|\begin{array}{ll}
{ 3} & {4} \\
{1} & {-2}
\end{array}\right| =-6-4=-10.
$$
