Oops, brain fart - Fm = iL x B, so Fm/L = i3*B(tot). B(tot) is equal to (μ0*√i1^2+i2^2) / (2PI*d), so Fm/L is equal to i3 times this.
For c, if the angle is to be 45 degrees, then tan^-1(i2/i1) = 45 degrees. This would mean that the ratio of i2/i1 is one to one, so i1 would have to be the same...