A string of length ℓ is fixed at both ends. It is vibrating in its 3rd overtone with maximum amplitude a. The amplitude at a distance ℓ/6 from one end is:
Step 1: 3rd overtone = 4ᵗh harmonic.
Step 2: Nodes occur at:
x = 0, (ℓ)/(4), (ℓ)/(2), (3ℓ)/(4), ℓ
Step 3: Since ℓ/6 lies very close to a node, amplitude is zero.