$($a$)(5$ marks$)$ In Figure $1,$ four long straight wires are perpendicular to the page, and their cross sections form a square of edge length $a=40$ cm $.$ The currents are out of the page in wires $1$ and $4$ and into the page in wires $2$ and $3,$ and each wire carries $12$ A $.$ In unit$-$vector notation, what is the net magnetic field at the square's center?

$($b$)(5$ marks$)$ In Figure $1,$ four long straight wires are perpendicular to the page, and their cross sections form a square of edge length $a=13\mathrm{.}5cm.$ Each wire carries $7\mathrm{.}50$ 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 $4?$