Start by drawing the region. You should find that your area of integration is the "top" of the parabola, from upwards and between and .

Now, if you were going to rotate this region about the line , you would have circular cross sections that have the same radius as the height of that region, so .

The area of each circular cross section is .

The volume will be the sum of all these cross sections over the region and . This sum converges on an integral.

So the integral you are evaluating is

.