Two wires have current both running in the positive x direction as shown in the diagram with I1 = 13.9 amps and I2 = 4.5 amps. They are separated by a distance of d = 4.7 meters. A third wire with I3 = 10 amps is placed somewhere between the two wires with current running in the positive x direction. How far above (in the positive y direction) above I1 should I3 be placed such that the force per unit length from both wires acting on I3 be equal to zero? Answer in meters.
http://theory.uwinnipeg.ca/physics/mag/node10.html
set the forces equal, solve for the distance.