Answer
$0.500$ m
Work Step by Step
The distance between adjacent wavefronts is defined as the wavelength. A formula relating velocity, wavelength, and frequency is the formula $$v=f \lambda$$ where $\lambda$ is the wavelength. Solving for wavelength and using the speed of sound as $343m/s$ yields $$\lambda = \frac{v}{f}=\frac{343m/s}{686Hz}=0.500m$$
