What frequency of sound has a wavelength of 75cm?

At first you have to convert your wavelength to meters:

75 cm = 0.75m

then you need to know the speed of sound - I've found that it is 340.29 m/s

and finally, you can use this equation:

λν = c

(0.75m) (f) = 340.29

f = 340.29 / 0.75

f=453.72 Hz