 #1
Homework Statement:
 A portion of along,cylindrical coaxialcable is shown in the accompanying figure. A current I flows down the center conductor, and this current is returned in the outer conductor. Determine the magnetic field in the regions (a) r ≤r1, (b) r2 ≥r ≥r1, (c) r3 ≥r ≥r2, and (d) r ≥r3. Assume that the current is distributed uniformly over the cross sections of the two parts of the cable.
Homework Equations:

B∫dl = μI
Inner current = I1
Outer Current = I2
I = J*A
J = Current Density
A = Area
I have no idea where to start.