In Fig. $29-57,$ four long straight wires are perpendicular to the page, and their cross sections form a square of edge length $a=13.5 \mathrm{~cm} .$ Each wire carries $7.50 \mathrm{~A},$ and the currents are out of the page in wires 1 and 4 and into the page in wires 2 and $3 .$ In unitvector notation, what is the net magnetic force per meter of wire length on wire $4 ?$