How do you find the volume of the solid formed by revolving the region bounded by the graphs of the equations #y=2x#, #y=4#, #x=0# using the shell method?

having trouble with this problem
I know that the shell method is: #V=2pi int_a^br(x)h(x)dx#
and also that r(x) is the distance between the axis of revolution and the center of the rectangle, and h(x) is the height of the rectangle.