Awave passes the dock every 5 seconds and reaches a height of .5 meters and a wavelength of 2 meters. find the wavespeed

We don't care about the height of the wave.
We're only interested in its period and wavelength, because
wave speed is the product of frequency and wavelength .

Period = 5 seconds
Frequency = 1/period = 1/5sec = 0.2/sec = 0.2 Hz.

Speed
= (frequency) x (wavelength)
= (0.2 Hz) x (2 m) = 0.4 m/s .