A solid conducting sphere with radius R carries a positive total charge Q

A solid conducting sphere with radius R carries a positive total charge Q. The sphere is surrounded by an insulating shell with inner radius R and outer radius 2R. The insulating shell has a uniform charge density ρ.
a) Find the value of ρρ so that the net charge of the entire system is zero.
b) If ρρ has the value found in part (a), find the electric field E→ (magnitude and direction) in each of the regions 0<r<R, R<r<2R, and r>2R. Graph the radial component of E→ as a function of r.
c) As a general rule, the electric field is discontinuous only at locations where there is a thin sheet of charge. Explain how your results in part (b) agree with this rule.