Question

What frequency of sound has a wavelength of 75cm?

Answer 1

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