The current density inside a long, solid, cylindrical wire of radius a = 3.4 mm is in the direction of the central axis and its magnitude varies linearly with radial distance r from the axis according to J = J0r/a, where J0 = 300 A/m2. Find the magnitude of the magnetic field at a distance (a) r=0, (b) r = 2.6 mm and (c) r=3.4 mm from the center.