Walker, Halliday & Resnick — Magnetic Fields Due to Currents: Problem 7

7 In Fig. 29-26, four long straight wires are perpendicular to the page, and their cross sections form a square of edge length \(a = 13.5 \text{ cm}\). Each wire carries \(7.50 \text{ A}\), and the currents are out of the page in wires 1, 3, and 4 and into the page in wire 2. In unit-vector notation, what is the net magnetic force per meter of wire length on wire 4?