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 .
