# Question: a dialectic sphere with radius r has a volumetric density...

###### Question details

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

Thank you