Answer
$406Hz $; $812Hz $
Work Step by Step
We know that
$\Delta l=l_2-l_1$
$\Delta l=(2.55+\frac{0.845}{2})-(2.55-\frac{0.845}{2})m=0.845m $
and $\lambda=\frac{\Delta l}{m}$
$\lambda=\frac{0.845}{m}$
Now $ n=\frac{\nu}{\lambda}$
$ n=\frac{343}{\frac{0.845}{m}}Hz $
$ n=406mHz $
where $ m=1,2,3,........$
for $ m=2$
$ n_2=406(2)=812Hz $
Thus, the two lowest frequencies are $406Hz $ and $812Hz $.