Calculate the frequency of when its wavelength is 0.85 m and the speed is 340 m/s.

The relationship between the velocity of the sound, its wavelength, and frequency is given by the formula:

 v=λ×T
where, v - velocity of the wave
           λ- wavelength of the wave
           f - frequency of the wave. 
In the question, it is given that the velocity of the wave is 340 m/s and the wavelength is 0.85 m. 
Now the frequency is calculated as follows:
$\mathrm{v}= \mathrm{\lambda f}$

$\mathrm{f}= \frac{\mathrm{v}}{\mathrm{\lambda }}= \frac{340}{0.85}$

$\mathrm{f}= 400 \mathrm{Hz}$