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 amplitude of the wave?

The distance between a trough and a crest is double the amplitude.

The distance between trough and crest is (9m - 6m) = 3m.

So the amplitude of those water waves is (3/2) = 1.5 meters.

For this question, we really don't need to know how far apart the crests are, or how often they pass.  There may be more parts to the question, for which that information is needed.

For example: 

-- What's the wavelength of the waves ?  8 meters.

-- What's the period of the waves ?  4 seconds.

-- What's the frequency of the waves ?  1 / (4 seconds) = 0.25 Hz.

-- What's the speed of the waves ? (8 m) x (0.25 Hz) = 2 m/s