How long must a simple pendulum be if it is to make exactly one swing per three seconds? (that is, one complete oscillation takes exactly 6.0 s.) ?

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