I agree with Prove It. But if this is just your professor being annoying and wants you to use Polar coordinates even when it is unnecessary, you can recall that for Polar coordinates
$\displaystyle x \mapsto r \cos \theta$
$\displaystyle y \mapsto r \sin \theta$
and
$\displaystyle dx~dy \mapsto r ~dr~d \theta$
Hopefully you can use this to figure it out.