Magnetic field of a Coaxial Cable

[evanclear]

Homework Statement

A long coaxial cable consists of two concentric conductors with dimensions a = 1.20 cm, b = 5.40 cm and c = 7.30 cm. There are equal and opposite currents of 5.00 A in the conductors. Find the magnitude of the magnetic field at a distance of 0.36 cm from the center axis.

Homework Equations

$\oint$B*dl=μ_0*I_enclosed

The Attempt at a Solution

Far as i can tell because the outer portion of the wire is not enclosed I enclosed for that portion of the problem is 0 so:

B=I*r$/$2∏a²

But I'm not getting the right answer.

Any help would be greatly appreciated

 

[mfb]

0.36cm is within the inner cable - just some fraction of the 5A current flow in the inner wire is enclosed in your integral.
You can assume that (electric) current density is constant in the wire.