Find the volume of the solid generated by revolving the region bounded by the graphs of the equation about the line y = 4.

I set y = y to find the boundaries.

I found the boundaries to x = o

and x = 3.

V = pi INT from x = 0 to x = 3 of the function (x)^2 dx.

My Answer: 9pi.

Textbook answer: 18pi.

Where did 18 come from?