in vibrations in string, the fundamental frequency happens to be the first harmonic, and is given as f\(_0\) = \(\frac{V}{2l}\) Where V = Velocity of the sound wave, I = Length of the string. The second harmonic is the first overtone given by f1 = \(\frac{V}{l}\) = 2f0 ; and the third harmonic happens to be the second overtone, given by f2 = \(\frac{3V}{2l}\) = 2f0. Thus if V = 120m/s and l = 1.5m Thus f1 = 2f0 = 2 x 40 = 80Hz Thus f2 = 3f0 = 3 x 40 = 120Hz Thus we have in order, 40Hz, 80Hz, 120Hz