It's a plane cutting through the circular cylinder of radius 1 at an angle. The integral is
Int in x from 0 to 1 of Int in y from -sqrt(1-x2) to sqrt(1-x2) of int in z from 0 to 2x dzdydx.
This ends up as integral from 0 to 1 of 2(1-x2) dx = 4/3
Please consider a tutor. Take care.