Use the shell method to evaluate the volume between y=(x)^(1/2), y=0, x=4, about the line x=6.
The formula is
$\displaystyle V = 2 \pi \int_a^b r f(x)\,dx$
where $\displaystyle f(x)$ is your function and $\displaystyle r$ the distance from a point in the region (along the x axis) to the line. So here
$\displaystyle V = 2 \pi \int_0^4 (6-x) \sqrt{x}\, dx$
I'm sure you can take it from here.