The current density inside a long, solid, cylindrical wire of radius a $=4\mathrm{.}6$ mm is in the direction of the central axis

and its magnitude varies linearly with radial distance r from the axis according to J $=$ Jor$/$a$,$ where Jo $=360$ A$/$m$2.$

Find the magnitude of the magnetic field at a distance $($a$)$ r$=0,($b$)$ r $=2\mathrm{.}5$ mm and $($c$)$ r$=4\mathrm{.}6$ mm from the center.