Question

Waves are observed passing under a dock. Wave crests are 8.0 meters apart. The time for a complete wave to pass by is 4.0 seconds. The markings on the post submerged in water indicate that the water level fluctuates from a trough at 6.0 meters to a crest at 9.0 meters. What is the wavelength of the wave?

Answer 1

To answer that question, we don't care what the highest and lowest
levels of the wave are, or how far apart they are.  We only need to be
able to identify the highest point on the wave, and keep track of how
often those pass by us.

You said it takes 4 seconds for a complete wave to pass by.
Through the sheer power of intellect, I'm able to take that information
and calculate that  1/4  of the wave passes by in 1 second.

There's your frequency . . .  1/4 per second, or  0.25 Hz.

Answer 2

Answer:

Wavelength = 8 m

Explanation:

As we know that wavelength is the distance moved by the wave in one time period of the wave.

Here we know that wavelength can be defined either by distance between two consecutive crest or two consecutive trough.

Here we know that it is given that crest on the wave is 8 meter apart which means the distance between two consecutive crest  is 8 meters.

So here distance traveled by the wave in one complete time period that is given as 4 s is wavelength of wave which is given here as 8 meter

so here we can say

$\lambda = 8 meter$