Answer
$1.2KHz$
Work Step by Step
We can find the lowest frequency as follows:
$d=\sqrt{(0.85)^2+(1.1)^2}=1.39m$
The wavelength is given as
$\lambda=\Delta d=1.39-1.1=0.29m$
Now we can find the frequency
$f=\frac{v}{\lambda}$
We plug in the known values to obtain:
$f=\frac{343}{0.290}=1.2KHz$
