$7)$ A long straight co$-$axial cable consists of a thin, inner wire of radius $\backslash ($ a $\backslash )$ and a thin outer shell at radius b$.$ A current $/$ flows into the page in the inner wire and out of the page in the outer shell.

Find the magnetic field $\backslash ($ B$($r$)\backslash )$ in the space between the wire and the shell as a function of the distance $\backslash ($ r $\backslash )$ from the central axis of the cable.