What is the potential difference between two concentric charged spheres?

[i_hate_math]

Homework Statement

In the figure, a metal sphere with charge q=https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2561214entrance1_N10031.mml?size=14&ver=1471825532267&algorithm=1&rnd=1471825539091 μC and radius r=https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2561214entrance1_N1003F.mml?size=14&ver=1471825532267&algorithm=1&rnd=1471825539091 cm is concentric with a larger metal sphere with charge Q=https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2561214entrance1_N1004D.mml?size=14&ver=1471825532267&algorithm=1&rnd=1471825539091 μC and radius R=https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2561214entrance1_N1005B.mml?size=14&ver=1471825532267&algorithm=1&rnd=1471825539091 cm. (a) What is the magnitude of the potential difference between the spheres? If we connect the spheres with a wire, what then is the charge on (b) the smaller sphere and (c) the larger sphere?

Homework Equations

V=k*(q/r)

The Attempt at a Solution

I got b and c correct, it is just part a that I am confused. The question ask for the potential difference between the two sphere, so i simply took the difference. ΔV=k*(Q/R - q/r)=7.8E5, please help me understand why this is not the correct answer. Thanks

 

[Paul Colby]

The potential change (difference) is the work it takes to move a unit of charge from one point to another. How would you calculate that work?