Answer
$579\;\rm Hz$, $4929\;\rm Hz$
Work Step by Step
Using the open-closed tube model, we know that the frequency is given by
$$f_m=\dfrac{mv}{4L}\tag{$m=1,3,5,..$}$$
The only thing that changes here is the speed of sound waves.
Hence,
$$f_{m,\rm new}=\dfrac{mv_{\rm new}}{4L}$$
where, from the first formula above, $4L=mv/f_m$, so
$$f_{m,\rm new}=\dfrac{v_{\rm new}}{v}f_m$$
For the first one,
$$f_{m,\rm new}=\dfrac{750}{350}\cdot270=\color{red}{\bf 579}\;\rm Hz$$
For the second one,
$$f_{m,\rm new}=\dfrac{750}{350}\cdot 2300=\color{red}{\bf 4929}\;\rm Hz$$
