Answer:
[tex]154.44\ \text{m/s}[/tex]
Explanation:
L = Length of guitar string = 0.66 m
Difference of fourth and first harmonic = 351 Hz
v = Wave propagation speed on the guitar strings
So,
[tex]\dfrac{4v}{2L}-\dfrac{v}{2L}=351\\\Rightarrow \dfrac{3v}{2L}=351\\\Rightarrow v=\dfrac{351\times 2\times 0.66}{3}\\\Rightarrow v=154.44\ \text{m/s}[/tex]
The wave propagation speed on the guitar strings is [tex]154.44\ \text{m/s}[/tex].