Change of Variables & the Jacobian · Medium problem

A solar panel is shaped like the ellipse $\dfrac{x^2}{16}+\dfrac{y^2}{9}\le1$ (meters). Using the substitution $x=4u$, $y=3v$ (which maps the unit disk to the ellipse, with Jacobian $12$), find the average value of $x^2+y^2$ over the panel.