Solved it. Thanks anyway.
Wait, how do you know the velocity of the wave? Doesn't that only work for a wave in air? Shouldn't you use a different value for the velocity on a string?
EDIT: It really doesn't matter, though. Everything but the length and frequency will cancel.
So if F = 440hz, then F1 = 440hz = xF2 = x(523hz), right? Where x is some constant to get 440 (don't want to calculate it =/ )
So then F1 = m(v/(2*30)) = xm(v/(2*L))
You know everything but L by now and everything else cancels.
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