How to find the solid generated by revolving a loop of the curve $\displaystyle r^2=a^2sin2 \theta$ about the initial line.

Here I used the fact that the volume

$\displaystyle V=\frac{2}{3}\int(f(\theta))^3sin\theta d\theta.$

But I couldn't get the answer. The answer should be $\displaystyle \frac{{\pi}^2 a^3}{8}$