Dielectric constant is throwing me off

[Ahmed.Sandman]

A spherical capacitor is formed from two concentric, spherical, conducting shells separated by vacuum. The inner sphere has radius 0.39 m and the capacitance is 108 pF. What is the radius of the outer sphere? Give your answer in meters to the third decimal place.

C-->Capacitance
K*C*rb-K*C*ra=ra*rb

it says its a vacuum so I'm using K=1

but I'm not getting a correct answer?!??!

 

[ehild]

K is not the relative dielectric constant but 1/(4∏_ε0, the constant in Coulomb's law.

http://www.phys.uri.edu/~gerhard/PHY204/tsl106.pdf

ehild