Answer
$$
71.5 \mathrm{Hz}
$$
Work Step by Step
Using Eq. $17-39$ and with $n=1$ and $343$ $\mathrm{m} / \mathrm{s}$ for speed of sound, we can get
$$
f=\frac{(1) v_{\text {sound }}}{4 L_{\text {the }}}=\frac{343 \mathrm{m} / \mathrm{s}}{4(1.20 \mathrm{m})}=71.5 \mathrm{Hz}
$$
