Solving Gaussian Problem: Cylindrical Shells w/ Radii R1 & R2

[Katsmed23]

Homework Statement

A thin cylindrical shell of radius R1 = 5.0 cm is surrounded by a second cylindrical shell of radius R2 = 9.0 cm, as in the figure. Both cylinders are 5.8 m long and the inner one carries a total charge of Q1 = +4.2 µC and the outer one carries a total charge of Q2 = -2.4 µC. (Assume the positive direction is away from the axis.)

a) r = 1.5 cm
N/C

(b) r = 5.5 cm
N/C

(c) r = 11.5 cm
N/C

Homework Equations

E *Da = Qenclosed/Epsilon

point charge = 1/4pi epsilon * Q/r^2

The Attempt at a Solution

a) radius is smaller than the radius of the first shell, so it's EF = 0
b) I've tried using E = Q / 4pi *epsilon *r^2, but i keep getting the wrong answer = 12.4e6 N/C
c) I think I should add the charges together (-2.4 + 4.2 = 2uC) and use the above equation, but I am at a loss.

I keep trying to find the answers to these last 2, but can't seem to get it right.

 

[Tuomaaca]

do you need to find the E-field at the given r's, at the midplane of the cylinders?

 

[Katsmed23]

"For points far from the ends of the cylinders, determine the electric field at the following radial distances from the central axis."

yes sorry forgot to include that!