Question

A longitudinal wave is observed. Exactly 6 crests are observed

to move past a given point in 9.1 s. Its wavelength is 2.4 m and
its frequency is 0.66 HZ. What is the speed of the wave?

Answer 1

The speed of the wave is 1.58 meters per second (or 1.584 if you don’t round it).
You multiply wavelength by frequency to get the speed of a wave.
2.4 times 0.66 = 1.548 meters per second.

If you didn’t have the frequency before hand, you would just divide the 6 crests by the 9.1 seconds.