When two sound sources of the same amplitude and similar frequencies f₁ and f₂ are played at a time, beats are produced with a frequency given by:
[tex]f_{beat}=|f_2-f_1|[/tex]Since the guitar player wants to hear 9 beats in 2 seconds, then:
[tex]f_{beat}=\frac{9}{2s}=4.5Hz[/tex]Then, the pitches that can be used are those whose difference with respect to the frequency of 343Hz is 4.5Hz:
[tex]\begin{gathered} f_2=f_1\pm f_{beat} \\ \\ \Rightarrow f_2=343Hz\pm4.5Hz \\ \\ \therefore f_{2,1}=347.5Hz \\ f_{2,2}=338.5Hz \end{gathered}[/tex]Therefore, the pitches he could bend the string to, are 347.5Hz and 338.5Hz.