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 period of the wave?

Concealed in all of that information, you told us that

           "The time for a complete wave to pass by is 4.0 seconds." 

That's the period of the wave right there ... The time for
a complete wave to pass by . . . . . 4 seconds.

We COULD calculate a lot of other things about the wave from
the other information you stuffed into the question, but you didn't
ask for other things.  You only used all of that other information to
distract and confuse us.