For a symmetrical wave like this*, the amplitude is half the difference between the highest and lowest values:
1/2(-1 -(-5)) = 2
The amplitude is 2.
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* There is no general agreement as to how to compute the amplitude when the wave is asymmetrical. Some authors use the same formula. Some consider the amplitude to be the maximum deviation from average. Some define only a "peak-to-peak" amplitude in those cases. The meaning of "amplitude" in those cases depends on the context in which the question is asked.
