Question 1113207


h(t)= {{{-16t^2+20t+84}}}

when it hits the ground h(t) =0

{{{-16t^2+20t+84}}}=0

solve the quadratic equation.

4t^2 -5t-21=0

4t^2 -12t+7t-21=0

4t(t-3) +7(t-3)=0
(t-3)(4t+7)=0

t= 3 
after 3 seconds