Generally, the definition of the speed is the distance per the time unit
Then, for one complete wave, the wave moves a distance equal to the wavelength (λ) in time equal to the period (T)
[tex]v=\frac{d}{t}=[/tex] λ/T
but we now that the frequency (f) is equal to the reverse of the period (T)
[tex]f=\frac{1}{T}[/tex]
Then the speed of the wave is determined by
[tex]v=[/tex] λ f
in our problem, we need to find the wavelength
note that: the speed must be in m/s not km/s , that you need to convert km to m by multiplyling by 1000
λ = [tex]\frac{v}{f}=\frac{13000}{2}=6500m[/tex]
