Two very long, parallel wires are separated by d $=0\mathrm{.}035$ m$.$ The first wire carries a current of I$1=0\mathrm{.}15$ A$.$ The second wire carries a current of I$2=0\mathrm{.}45$ A

Express the magnitude of the force between the wires per unit length, f$,$ in terms of I$1,$ I$2,$ and d$.$