How can we solve this 3rd degree equation where the unknown is $\displaystyle x\ge0 $ and a,y constants

$\displaystyle \frac{x^3}{9a^2}-\frac{2x^2}{3}+a^2x-y^2=0$

The above equation comes from the main problem which is to express this formula $\displaystyle y=(\frac{x}{3a}-a)\sqrt x$ as term of x

Is there any other way to express the second formula as term of x except to solve the first 3rd grade equation??

Thanks anyone in advance