Rock will hit the ground after a time of 6.45 s
initial velocity=20 m/s
initial displacement=Yo=75m
final displacement=0
using the kinematic equation
Y= Yo+ V t+ 1/2 gt²
0=75+20 t+1/2 (-9.8)t²
-4.9t²+20 t+75=0
This is a quadratic equation in t,s we use the quadratic formula
t=[tex] \frac{-20+-\sqrt{20^{2}-4(-4.9)(75)}}{2(-4.9)} [/tex]
t=6.45 s
