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