Multiply both sides by 2pi:
F(2pi) = sqrt(g/l)
Square both sides:
F^2 (2pi)^2 = g/l
Solve for l by dividing both sides by F^2 (2pi)^2
L = g/ F^2 (2pi)^2
Simplify:
L = g / 4pi^2(f^2)
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