The equation to use for this would be:
Wave speed (m/s) = frequency(Hz) * wavelength (m)
The question has given a wavelength, however, it is in mm so we will have to convert it into m as wavelength is always in meters:
13mm -> 0.013m
Now we can work out the rest:
Wave speed = 6 * 0.013
Wave speed = 0.078 m/s <-- answer
