I solved it I forgot to square the sin at the end. I did't need to use a u substitution.
Any regardless if you have that bound it yields a negative answer.
Thanks for helping though.
The answer is
\pi a^4/4
So the code is messed up basically
Integral from 0 to a [4x^2 squareroot(a^2 - x^2)]dx =
4a^4 integral from 0 to pi/2 [sin theta cos^2 theta] dtheta
I'm stuck trying to change the bounds with a u substitution