Finding the volume of a solid by rotating two curves about the $y$-axis Find the volume of the solid obtained by rotating the region bounded by the given curves about the $y$-axis.
$$y=56x−7x^2 \mbox { and }y=0$$ 
My issue with this question is that I am having trouble turning the equation, $y=56x−7x^2$, in terms of $y$. I understand that by doing that, I can proceed with the integration...
Perhaps there is another method to do this without having to turn it in terms of $y$?
Thanks

Divide everything by $7$, giving
$$\frac{y}{7} = 8x - x^2.$$
Multiply by $-1$ and complete the square:
$$16 - \frac{y}{7} = (x-4)^2, \text{therefore } x = 4 \pm \sqrt{16 - \frac{y}{7}}.$$
Let me know if you have problems taking from here. Best wishes.
