physics help!?

[kagekiri_tora]

A solid conducting sphere of radius 2.00 cm has a charge 7.70 µC. A conducting spherical shell of inner radius 4.00 cm and outer radius 5.00 cm is concentric with the solid sphere and has a charge -2.60 µC. Find the electric field at the folowing radii from the center of this charge configuration.

I know that when r=1.00 cm that the answer is 0 N/c
and that when r=4.50 cm the answer is also 0 N/c

but I can't seem to get what N/C is when R=3.00 Cm and when R=7.00 Cm

[rijim2001]

You have to use Gauss' Law to find the E field at r= 4.5 cm and r = 7 cm. I'm sure that was what your chapter was about. You better read the chapter.
You should know :
Electric Field of Point Charge
Electric Field of Conducting Sphere
Electric Field: Sphere of Uniform Charge
Inside a Sphere of Charge
Electric Field: Sphere of Uniform Charge
SEE THE SOURCE BELOW

I will give you the answers, but you will have to do the work

E at r = 4.5 cm
E = 3.66 x 10^-14 volts / m

E at r = 7 cm
E = 9.21 x 10^-15 volts / m