Magnetic force between 3 straight wires

[kiewelb]

Find I2 so that the magnetic force per unit length on each wire is zero. The three parallel wires are equally spaced. Current directions are shown with '<' or '>'.

I1 = I3 = 1 A

< I1 < I1 < I1
---------------
> I2 > I2 > I2
---------------
< I3 < I3 < I3
---------------

I have the formula F = (mu0*I*I')/(2*pi*r) with mu0 = 4*pi*10^-7 but have no clue where to take this to figure out I2 with the 3 wires.

Sorry if this is really easy but I'm not too quick with Physics. Thanks for any help.

[gnome]

r in the formula is the distance between 2 wires. So let r = distance between wire 1 and wire 2. Then the distance between wire 1 and wire 3 is ______?

Next, use that formula to find the force exerted on wire 1 by wire 2, and the force exerted on wire 1 by wire 3. If the NET force on wire 1 is 0, those two forces must be equal in magnitude & opposite in direction so set the two F's equal to each other & solve for I2.