In Fig. $29-57,$ four long straight wires are perpendicular to the page, and their cross sections form a square of edge length
$a=8.50 \mathrm{cm} .$ Each wire carries 15.0 $\mathrm{A}$ , and all the currents are out of
the page. In unit-vector notation, what is the net magnetic force per
meter of wire length on wire 1?