Answer
460 Hz
Work Step by Step
The wave speed is given by $v=\lambda f$. The speed is given as 344 m/s. Solve for the frequency at which the wavelength is 0.75 m.
$$f =\frac{v}{\lambda }=\frac{344 m/s}{0.75m}\approx 460Hz$$
Diffraction through the window occurs for wavelengths that are larger than the window width, which correspond to waves with frequencies less than about 460 Hz.
