Question: a dialectic sphere with radius r has a volumetric density...
A dialectic sphere with radius R has a volumetric density that is not uniform and is given by ρ=ρ0 r, where ρ0 is a positive constant. The sphere is enclosed by a spherical shell with a radius of 2R and the density of the superficial charge is σ, as shown in the figure. Obtain the value of σ so that the electric field outside of the shell (r>2R) is cero.
So the answer is the one in top of the pic but I need help to get there