The frequency of the nth-harmonic (corresponding to the (n-1)-th overtone) of a vibrating string is given by
[tex]f_n = n f_1[/tex]
where [tex]f_1[/tex] is the fundamental frequency of the string.
In our problem, [tex]f_1=256 Hz[/tex] is the fundamental frequency. The second overtone corresponds to the third harmonic, so by substituting n=3 into the previous equation, we find the frequency of the second overtone:
[tex]f_3=3 \cdot f_1 = 3 \cdot 256 Hz =768 Hz[/tex]
