A very long conducting tube (hollow cylinder) has inner radius a and outer radius b. It carries charge per unit length +alpha , where alpha is a positive constant with units of C/m. A line of charge lies along the axis of the tube. The line of charge has charge per unit length +alpha .
What is the charge per unit length on the inner surface of the tube?
What is the charge per unit length on the outer surface of the tube?
I've found the electric field where r < a, a < r < b, r > b already but I'm not sure how to apply that to the follow questions.