It's hard to tell exactly what's happening in that 110 cm that you marked over the wave. What is under the ends of the long arrow ? How many complete waves ? I counted 4.5 complete waves ... maybe ?
If there are 4.5 complete waves in 110cm, then the length of 1 wave is (110/4.5)=24.44cm.
Frequency = speed/wavelength
Frequency = 2m/s /0.2444m
Frequency = 8.18 Hz
