The current density inside a long, solid, cylindrical wire of radius a$=3\mathrm{.}4$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$\_0=260$A $/$ m$^2.$ Find the magnitude of the magnetic field at a distance $($a$)$ r$=0,($b$)$ r$=1\mathrm{.}9$mm and $($c$)$ r$=3\mathrm{.}4$mm from the center. $($a$)$ Number Units $($b$)$ Number Units $($c$)$ Number$^-$ Units