The wavelength of a sound wave is related to its frequency by the relationship:

where
f is the frequency
v is the speed of the wave

is the wavelength
The wave in our problem has wavelength of

and speed of

(this is the speed of sound in air), therefore its frequency is

And the period of the wave is equal to the reciprocal of its frequency:
Answer:
the speed of the textbook just before it hits the floor is 2.4 m/s
Explanation:
Given the data in the question;
mass of pulley = 1.50 kg
radius of pulley = 0.240 m
mass of text book = 2.0 kg
height from which text book was released = 0.9 m
angular speed of the pulley = 10.0 rad/s
the speed of the textbook just before it hits the floor = ?
the speed of the textbook v = angular speed of the pulley × radius of pulley
we substitute
v = 10.0 rad/s × 0.240 m
v = 2.4 m/s
Therefore, the speed of the textbook just before it hits the floor is 2.4 m/s