OHH! Okay okay! I think I get it!
Sorry for the many mistakes in values and units, I looked back in the textbook and corrected everything.
μo = 4π*10-7 Tm/A
density value is 8960 kg/m3
And then with 2*Fcos(30) = ρπr2*length*g, I can cancel lengths, the "2", and put it back into the other...