d
Electric field
$E=-\frac{d \phi}{d r}=-2 a r.........(i)$
By Gauss's theorem
$E=\frac{1}{4 \pi \varepsilon_{0}} \frac{q}{r^{2}}.........(ii)$
From $(i)$ and $(ii)$,
$\mathrm{q}=-8 \pi \varepsilon_{0} a r^{3}$
$\Rightarrow \quad d q=-24 \pi \varepsilon_{0} a r^{2} d r$
Charge density, $\rho=\frac{d q}{4 \pi r^{2} d r}=-6 \varepsilon_{0} a$