Answer
$f_1=49Hz$
Work Step by Step
The frequency of an air column is equal to $$f_n=\frac{nv}{2L}$$ Since the tone is the fundamental frequency, $n=1$. This means that $$f_1=\frac{v}{2L}$$ Substituting known values of the speed of sound $v=343m/s$ and $L=3.5m$ yields a frequency of $$f_1=\frac{343m/s}{2(3.5m)}=49Hz$$
