A hiker shouts towards a vertical cliff 465 m away. The echo is heard 2.75 s later. The wavelength of the sound is 0.750 m. Whats the frequency...

The velocity of a sound wave is calculated by multiplying
the frequency of the wave by the wavelength.

v = fl   where
f is the frequency in Hz and l is the wavelength in
meters.

In this case, you should first find the velocity of
the wave, then use that to find the frequency. From the frequency you can then calculate
the period.

So, lets find the velocity of the sound
wave.

velocity = distance/time.  You know the distance =
465m, and the time it takes to get to the cliff is 1/2 the time it takes for the shout
to hit the cliff and return.  So the time is 2.75/2  = 1.375
s

therefore the velocity of the wave is 465 m/1.375s =
338.2 m/s

Now use that velocity along with the wavelength
given to find the frequency of the sound.

338.2 = .750 m *
f

then f = 338.2/.75 = 450.9
Hz

Sinc T = 1/f, then T = 1/450.9=
0.0022s