y=3 is the axis of symmetry, hence imagine it made from a series of vertical discs
(you know what a tornado looks like!).
The radius of each disc is the distance from the function to the line y=3.
Hence the volume of the conical part is the integral from x=0 to x=9 of the disc areas.
Then subtract that answer from the volume of the external cylinder of radius 3 and height 9.
Think of the graph of the function from x=0 to x=9 as a line which is then made spin around the axis y=3.