A question about conducting sphere using Gauss's Law

[ehabmozart]

Homework Statement

A conducting spherical shell with inner radius a and outer radius b has a positive point charge Q located at its center. The total charge on the shell is -3Q, and it is insulated form it surroundings. What is the surface charge density on the inner surface of the conducting shell.? What is the surface charge density on the outer surface?

Homework Equations

EA= Q enclosed / ε0

The Attempt at a Solution

I know that since there is a point charge, this will change up the configuration a bit making the inner surface having -Q and outer surface to have -2Q charge enclosed. I have problems in determining the area. I don't know when does the inner end and when does the outer surface begin. Is it half way?? What would be the area then?

 

[ehild]

The radii of the inner and outer surfaces are given, a and b. Do you know how to calculate the area of a spherical surface from the radius?

ehild