The principle of superposition states that the resultant wave is the sum of displacement from equilibrium of each wave.
The amplitude of the wave is the maximum displacement from equilibrium, which can be read off the graph as $7\,cm−5\,cm=2\,cm$.
Since each of the waves have an amplitude of $2\,cm$ and the equilibrium water level is $5\,cm$, after superimposing the two waves we find that the amplitude of the resultant wave is $4\,cm$.
Therefore, the maximum and minimum water levels can be determined as: 5 cm+4 cm=9 cm for the maximum level and 5 cm−4 cm=1 cm for the minimum level.
