Answer:
2 seconds
Step-by-step explanation:
f = Frequency of yell = 919 Hz
= Shifted frequency = 55.9 Hz
v = Speed of sound in air = 342 m/s
= Velocity of friend
a = Acceleration due to gravity = 9.81 m/s²
From the Doppler shift formula we have

The velocity of the my friend is 19.61 m/s

The time my friend is in the air is 2 seconds