Answer
70 Hz
Work Step by Step
The frequencies of the harmonics for a string that is fixed at both ends are given by equation 11-19b.
$$f_n=nf_1$$
Two successive overtones will differ by the fundamental frequency $f_1$.
$$\Delta f=f_{n+1}-f_n=(n+1)f_1-nf_1=f_1$$
$$f_1=350Hz – 280Hz = 70 Hz$$
