A sound wave of the form s = s_m cos(kx - t + ) travels at 344 m/s through air in a long horizontal tube. At one instant, air molecule A at x = 2.02 m is at its maximum positive displacement of 6.10 nm and air molecule B at x = 2.10 m is at a positive displacement of 2.20 nm. All the molecules between A and B are at intermediate displacements. What is the frequency of the wave?