The distance between two consecutive points with phase difference...

Distance between two consecutive points $60 °$ out of phase is given by $\Deltax = \dfrac{\lambda}{2\pi}\Deltaϕ$

Putting the values, we have

$6 = \dfrac{\lambda}{2\pi}\left(\dfrac{\pi}{3}\right)$

$⇒ \lambda = 36m$

Now, velocity of wave is $v = f\lambda = ( 500Hz ) ( 36m )$

$= 18000m{s}^{- 1} = 18km{s}^{- 1}$