The charge desnsity within a uniformly charged sphere of radius R is ρ = ρ0 - ar2, where ρ0 and a are constants and r is the distance from the center. Find an expression for a such that the electric field outside the sphere is zero.
Electric Field outside a sphere
E = KQ/r2
The Attempt at a Solution
I'm having trouble setting up this problem, I'm not quite sure where to start. If someone could help me I would very much appreciate it.