The current density inside a long, solid, cylindrical wire of radius a$=4\mathrm{.}8$ mm is in the direction of the central axis and its magnitude varies linearly with radial distance r from the axis according to J$=$J$0$r$/$a$,$ where J$0=370$ A$/$m$^2.$ Find the magnitude of the magnetic field at a distance $($a$)$ r$-0,($b$)$ r$-3\mathrm{.}3$ mm and $($c$)$ r$=4\mathrm{.}8$ mm from the center.