Answer
$170\;\rm Hz$
Work Step by Step
At your initial position, there is a maximum sound intensity, so
$$\Delta r=m\lambda$$
At your final position, there is the first minimum sound intensity, so
$$\Delta r=r_2-r_1=\dfrac{\lambda}{2}$$
where $r_1=12$ m, and $r_2=\sqrt{5^2+12^2}$, and $\lambda=v/f$,
$$ \sqrt{5^2+12^2}-12=\dfrac{v}{2f}$$
Solving for $f$,
$$f=\dfrac{v}{2[\sqrt{5^2+12^2}-12]}$$
$$f=\dfrac{340}{2[\sqrt{5^2+12^2}-12]}$$
$$f=\color{red}{\bf 170}\;\rm Hz$$
