Wavelength is the distance between 2 adjacent points in a wave
we can use the following equation to find the wavelength of a sound wave
wavelength = speed / frequency
frequency is the number of waves passing a point in 1 second
substituting the values in the equation
wavelength = 343 m/s / 686 Hz
wavelength = 0.5 m
wavelength of the wave is 0.5 m