Question 1188509
The formula is s(t)=-16t^2+48t+64
the ground is 0.
so -16t^2+48t+64=0=t^2-3t-4, moving everything over and dividing by 16
(t-4)(t+1)=0
t=4 seconds.
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want to know when s=64
so -16t^2+48t+64=64
or 16t^2=48t
t=3 seconds 
{{{graph(300,300,-5,5,-60,150,64,-16x^2+48x+64)}}}