I'm having trouble visualizing and putting together this concept.

Question: Find volume of solid generated by revolving the region bounded by $\displaystyle y=x^3-4x$ and $\displaystyle y=2-x$ about the line $\displaystyle x=-1$

So I want to use vertical cylindrical shells to find the volume. I know the general formula for a cylindrical volume is V=circumference x height x thickness. So i know it should be $\displaystyle 2\pi(x+1)$ for circumference and $\displaystyle dx$ for thickness. But i dont see hows its $\displaystyle (2-x)-(x^3-4x)$ for height.

Thanks