the pitch that the average human can hear has a frequency of 20.0. If sound with this frequency travels through air with the speed of 331 m/s, what is its wavelength.

There are problems with the first sentence, and it's not really needed when working with this question.  So let's just take the 20 (Hz ?) frequency from the first sentence, and ignore the rest of it for right now.           Wavelength = (speed) / (frequency) =                                   (331 m/s) / (20 Hz) = 16.55 meters.