For a charged spherical ball, electrostatic potential inside the ball varies with r as $\mathrm{V}=2 a \mathrm{r}^2+\mathrm{b}$. Here, a and $b$ are constant and $r$ is the distance from the center. The volume charge density inside the ball is $\lambda \mathrm{a} z$. The value of $A$, is $\_\_\_\_$ . $\varepsilon=$ permittivity of medium.