Amplitude is defined as the maximum distance that is moved by a point on a wave which is measured from its equilibrium position. It is equal to the half the length of the vibration path.
In the given figure,
OA is the one half of the distance of the vibration pathway and is the amplitude of the wave.
The amplitude is from the absolute value of the 0 point on the y-axis to the highest(peak) or lowest(troph) point of the wave. In this question, 3cm is the highest and -3cm is the lowest, so the amplitude is 3cm.
