Three parallel current-carrying wires are shown in Figure P30.67. The currents in each wire are I1 = 0.500 A, I2 = 0.750 A, and I3 = 0.250 A. If the wires are separated by distances r1= 0.185 m and r2 = 0.290 m, find the net magnetic field force per unit length on the middle wire due to the other wires.