107. assignment (wave frequency)

A sound wave of the form s=sm*cos(k*x-omega*t + phi) travels at 345 m/s through air in a long horizontal tube. At one instant, air molecule A at x=2.01 m is at its maximum positive displacement of 6.00 nm and air molecule B at x=2.11 m is at a positive displacement of 2.40 nm.

 

All the molecules between A and B are at intermediate displacements. What is the frequency of the wave?
 
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