Answer
$0.31KHz$
Work Step by Step
We can find the required frequency as follows:
$d_1=\sqrt{(5.0m)^2+(\frac{1}{2}(3.5m+0.84)^2}$
$d_1=5.631m$
and $d_2=\sqrt{(5.0m)^2+(\frac{1}{2}(3.5m-0.84)^2}$
$d_1=5.082m$
Now $d_1-d_2=\frac{1}{2}\lambda$
$\implies \lambda=2(d_1-d_2)$
$\lambda=2(5.631m-5.082m)=1.098m$
The frequency is given as
$f=\frac{v}{\lambda}$
We plug in the known values to obtain:
$f=\frac{343}{1.098}$
$f=0.31KHz$
