The volume of this solid can also be computed using cylindrical shells via an integral

2pi(x+4)(4x-x^2) dx

with limits of integration 0 and 4

volume is cubic units.

I am stuck on the last step. For some reason I tried integrating it and then plugging in the value, which in this case is just the beta, 4.

What am I doing wrong? this is supposed to be easy step..