Net charge on sphere

Suppose that a=4 cm, b=21 cm, and c=28 cm. Furthermore, suppose that the electric field at a point 11 cm from the center is measured to be 2420 N/C radially inward while the electric field at a point 63 cm from the center is 130 N/C radially outward. Find the charge on the insulating sphere.
I'm sorry I can't post the picture, but it is an insulator with a radius 4 cm, that is inside of another sphere with a radius of 21 cm measured from the center, which is inside of a conductor with a radius of 28 cm measured from the center.
We did part of this problem in class and found that the $$Q_e_n_c = E_1 *4\pi* R_g^2 *E_0.$$
I'm pretty sure that E_1 = 2420 N/C and E_0= 8.85 x 10^-12, but I'm a little confused about which radius to use. I thought it would be the radius of the insulator which is .04 m, but this wasn't right. Can someone please help?

Astronuc
Staff Emeritus
E_0= 8.85 x 10^-12 C2/N-m2 is just the electric permittivity of free space. It is important to realize the units. I prefer using $\epsilon_o$.