the distance between two wave crests in the water is 3 meters. a swimmer notices one wave goes by every 6 seconds. What is the spped of the waves

Answer 1

distance = speed * time
3 = v * 6
v = 1/2 meter /second

note - that is called the "phase speed" and is the observed speed of a wave crest. It is not the speed of the propogation of the wave system called "group velocity".