two circular coils are concentric and lie in the same plane. the inner coil contains $120$ turns of wire, has a radius of $0\mathrm{.}012$ m$,$ and carries a current of $7\mathrm{.}9$ A$.$ the outer coil contains $140$ turns and has a radius of $0\mathrm{.}021$ m$.$ what must be the magnitude of the current in the outer coil, such that the net magnetic field at the common center of the two coils in zero?