Answer
$0.18KHz$
Work Step by Step
We can find the required frequency as follows:
$d=\sqrt{(1.5)^2+(3.0)^2}$
$d=3.3m$
We know that
$\lambda=3.3m-1.5m=1.8m$
We know that
$f=\frac{c}{\lambda}$
We plug in the known values to obtain:
$f=\frac{340m/s}{1.8m}$
$f=0.18KHz$