the following figure shows a wave of frequency 50Hz. find from graph : ( a) the amplitude (b) the wavelength (c) the velocity of wave.

amplitude is the disturbance of the wave from mean position. therefore it is 3. (between 2 and 4 in y axis).
wavelength is the distance between two consecutive crust or two consecutive troughs. therefore the wave length is 20 (25-5 = 20)
velocity of wave = wavelength * frequency ( v = lambda * frequency)
                            = 20 *50 = 100 m/s