S is given by $z = \sqrt{x^2+ y^2} \ $ for $1 ≤ z ≤ 4$, then calculate $\nabla \cdot \vec{F} \ $ for the vector field $\vec{F} = (xz, −zy, x^3)$.

Evaluate $I = \iiint \ (\nabla \cdot \vec{F}) \ dV$ ,
where $V$ is the volume enclosed by the surface S and the two discs
{(x, y, z) : x^2+y^2 ≤ 1 , z = 2} and {(x, y, z) : x^2+y^2 ≤ 9 , z = 3}$.

I got a negative answer it thet right?