The formula for the volume of any function rotated around
is given by
where a and b are the limits of integration, R (outer radius) is the distance from the axis of rotation to the further side of the elemental strip and r (inner radius) is the distance from the axis of rotation to the closer side of the elemental strip. In your case, since the axis of rotation is the x-axis, which is
, your r is simply 0. This will cancel out and your formula will become
.
Your R (outer radius) is simply the function because your axis of rotation is the x-axis.
Your answer becomes:
Hope I helped
Do you get it now?