You won't $be$ able $to$ evaluate either $of$ the following two integrals. But, prove that they are

equal. $(Hint$: A Jacobian may $be$ involved$)$

${\displaystyle \underset{0}{\overset{2}{}}}{\displaystyle \underset{0}{\overset{x}{}}}2e^{x^2-y^2}dydx,{\displaystyle \underset{0}{\overset{2}{}}}{\displaystyle \underset{y}{\overset{4-y}{}}}{e}^{xy}dxdy$