[tex]\vec f(x,y,z)=y\,\vec\imath-4yz\,\vec\jmath+3z^2\,\vec k[/tex]
[tex]\implies\nabla\cdot\vec f(x,y,z)=0-4z+6z=2z[/tex]
By the divergence theorem,
[tex]\displaystyle\iint_{\partial W}\vec f\cdot\mathrm d\vec S=\iiint_W2z\,\mathrm dV[/tex]
I'll assume a sphere of radius [tex]r[/tex] centered at the origin, and that [tex]W[/tex] is bounded below by the plane [tex]z=0[/tex]. Convert to spherical coordinates, taking
[tex]x=\rho\cos\theta\sin\varphi[/tex]
[tex]y=\rho\sin\theta\sin\varphi[/tex]
[tex]z=\rho\cos\varphi[/tex]
Then
[tex]\displaystyle\iiint_W2z\,\mathrm dV=\int_0^{\pi/2}\int_0^{2\pi}\int_0^r2\rho^3\cos\varphi\sin\varphi\,\mathrm d\rho\,\mathrm d\theta\,\mathrm d\varphi=\pi r^4[/tex]
