A solid conductor with radius a is supported by insulating disks on the axis of a conducting tube with inner radius b and outer radius c $($Fig$.$ E$28\mathrm{.}39).$ The central conductor and tube carry equal currents I in opposite directions. The currents are distributed uniformly over the cross sections of each conductor. Derive an expression for the magnitude of the magnetic field $($a$)$ at points outside the central, solid conductor but inside the tube $\backslash (($ac$)\backslash )$

when r$>$c$,$ is the I$($enclosed$)2$I or $0?$