Use the disk method
take a small segment dx, and when you revolve the indicated region around the x-axis, the small segment will become a disk.
Therefore, you need the volume of a disk which is pi x r^2 x h, where r is the radius of the disk and h is the height.
if you look at the region, you will see that the radius = f(x) = 1/(x^2 + 1) while the height is dx.
And the integral will become