Three long parallel wires each carry 7 a currents in the same direction. the wires are oriented vertically, and they pass through three of the corners of a horizontal square of side 9 cm. what is the magnitude of the magnetic field at the fourth (unoccupied) corner of the square due to these wires? give your answer in µt. (µ₀ = 4p × 10⁻⁷ t · m/a)