Two ideal solenoids of radii R and 3R, respectively, have n turns per meter and each carries a current I. Both currents flow in the same direction. The small radius solenoid is placed inside the large radius solenoid so that their axes of symmetry are parallel and separated by a distance d = 2R . Neglecting any magnetic screening effects, express anaytically the magnetic field strength on the axis of the small-radius solenoid in terms of the quantities given above and the magnetic permeability of a vacuum.