Since you are rotating around the axis, this will be a integral.
The graphs intersect where the equations are equal...
When , so they intersect at .
The volume is calculated by rotating the area around the axis.
This area needs to be thought of as a series of rectangles, which when rotated, become cylinders.
The length of each rectangle is and the width is , a small change in .
When rotated, each cylinder has a radius the same as the length of each rectangle, so the circular cross-sectional area is . The height of each cylinder is the same as the width of each rectangle, .
Therefore the volume of each cylinder is .
So the entire volume can be approximated by , and when you increase the number of cylinders, this sum converges on an integral and the approximation becomes exact.