6. A system of electric charges has a charge density $\rho(x, y, z)$ and produces an electrostatic field $E(x, y, z)$ at points $(x, y, z)$ in space. Gauss' Law states that
$\iint_S E \cdot dS = 4\pi \iiint_D \rho dV$
for any closed surface S which encloses the charges, with D being the solid region enclosed by S. Show that $\nabla \cdot E = 4\pi \rho$.
