I don't really understand this question:

Evaluate:

$\displaystyle \int_0^3 x \sqrt{81-x^4}\, dx$

by making a substitution of the form $\displaystyle u=x^p$ and then interpreting the result in terms of a known area.

I get that I should find a usable $\displaystyle u=x^p$ then evaluate but don't understand what it means by interpreting in terms of a known area.

Thanks