$*$ P$16$ A negative point charge $-Q$ is at the center of a hollow insulating spherical shell, which has an inner radius $R_1$ and an outer radius $R_2.$ There is a total charge of $+3Q$ spread uniformly throughout the volume of the insulating shell, not just on its surface. Determine the electric field for $($a$)R_2{R}_2.R_1{R}_2,$ and $(c)R_2.r,(b)R_1{R}_2,$ and $(c)R_2.$