May I re-write the question as I understand it?:
Determine the volume when the area, enclosed by the x-axis and the graph of , rotates about the x-axis.
The volume is composed by cylindrical discs whose radius is y and whose height is dx. The volume of one of these discs is calculated by:
The complete volume of the solid is the sum of all those discs:
Plug in the term of y and determine the borders of the area and you'll get the equation skeeter has posted.