Use the diagram. Look at how far it is between one wavelength for the solid line, and look at how many squares the dashed line has passed after 25.
It looks like the wavelength is 2 metres, and the dashed line has passed 0.5 metres in 25 seconds. So the time period will be given by 25*4 = 100 seconds
Then Frequency is 1/Time Period = 1/100 Hz.
Speed = Frequency * Wavelength
Speed = 1/100 * 2 = 1/50 m/s