What is the Capacitance and Potential Difference of a Coaxial Cable?

[christianerik]

A 31 m length of coaxial cable has a solid
cylindrical wire inner conductor with a di-
ameter of 2.042 mm and carries a charge of
7.32 μC. The surrounding conductor is a
cylindrical shell and has an inner diameter of
6.748 mm and a charge of −7.32 μC.
Assume the region between the conductors
is air. The Coulomb constant is 8.98755 ×
109 N · m2/C2.
What is the capacitance of this cable? An-
swer in units of nF.

What is the potential difference between the
two conductors? Answer in units of kV

need help, I tried it but failed to get the right answer

 

[Donaldos]

$$C=L\frac{2\pi \epsilon_0}{\ln \frac D d}$$

D=6.748 mm, d=2.042 mm and L=31m.

Then use:

$$Q=CU$$