Answer
$$26.4 \mathrm{Hz}
$$
Work Step by Step
We know (wavelength of wave with low resonant frequency) $f_{1}$ is $$\lambda_{1}=2 L,$$ where $L =125 \mathrm{cm} .$
$\text{Thus,}$
$$
f_{1}=\frac{v}{\lambda_{1}}=\frac{66.1 \mathrm{m} / \mathrm{s}}{2(1.25 \mathrm{m})}=26.4 \mathrm{Hz}
$$
