A long solenoid with 1000 windings per meter is placed with its central axis parallel to the \(y\) axis and \(20 \mathrm{~mm}\) above a sheet of metal that is \(0.5 \mathrm{~m}\) wide in the \(y\) direction and carries a uniform current of \(20 \mathrm{~A}\) in the positive \(x\) direction. What current must the solenoid carry in order that the magnetic field at its centre is zero?