This is a physics question.
Use the formula for simple pendulum:
T = 2π √(L/g)
Where T is the period (time to complete a full oscilation), L is the lenght and g acceleration of gravity ≈ 9.81 m/s^2 .
So, solve for l:
√(L/g) = T / (2π)
L = [T / (2π) ] ^2 * g = [ 6.0s / (2π) ]^2 * 9.81 m/s^2 = 8.9 m
Answer: 8.9 m
