We can use the following formula:
[tex]v=\lambda f[/tex]
In wich [tex]v[/tex] is the speed, [tex]\lambda[/tex] is the wavelenght and [tex]f[/tex] is the frequency. Using the SI units:
[tex]v=\lambda f\\\\
\lambda = \dfrac{v}{f}\\\\
\lambda = \dfrac{6~\text{km/s}}{9~\text{Hz}}\\\\
\lambda = \dfrac{6000~\text{m/s}}{9~\text{Hz}}\\\\
\boxed{\lambda \approx 666.67~m}[/tex]
Hence, the wavelenght is approximately 666.67 m
