I need help on this problem like now

Question

A rock is thrown upward with a velocity of 24 meters per second from the top of a 20 meter high cliff, and it misses the cliff on the way back down. When will the rock be 7 meters from the water,

Answer 1

x_f - x_i = v_it + (1/2)at²   (kinematics equation)

 

x_f = 7

x_i = 20

v_i = 24

a = -9.8

 

7 - 20 = (24)t - (0.5)(9.8)t²

 

Rewrite in standard form:

 

4.9t² - 24t - 13 = 0

 

Solve the quadratic equation:

 

t = 5.39 seconds    or     t = -0.49 seconds

 

Choose the positive solution:

 

t = 5.39 seconds