$.$ Consider an infinitely$-$long wire carrying a current $$

a$.$ Find an expression for the magnetic field, $,$ of the wire as function of the distance from the wire. $(10$ points$)$

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b$.$Consider four infinitely$-$long wires, each carrying a current $,$ where each wire is situated so that it passes through one of the four corners of a square.$$

In the plane defined by $,$ the coordinates of the wires are

Find the magnetic field at the origin, $?($show your work$)(10$ points$)$