The electric field in a certain region is given by $\vec{E}=a x \hat{i}$, where $a=32 \mathrm{~N} / \mathrm{C} \cdot \mathrm{m}$ and $x$ is in meters. Find the volume charge density in the region. (Hint: Apply Gauss's law to a cube $1 \mathrm{~m}$ on a side.)