Potential Difference b/w Sphere & Cond. Shell | Electrostats

[hellboydvd]

From Electrostatics:

A solid conducting sphere of charge Q is surrounded by an uncharged concentric conducting spherical shell. The potential difference between the sphere and the shell is V. If the shell is now given a charge of -3 Q , what is the new potential difference between them?

 

[dipstik]

you need to read the rules and post this in the HW section (this might get moved)

show what you have done and tell us where you are getting lost. you might want to begin with V=integral of E wrt r. use some gauss law stuff.