check the picture below.
[tex]\bf \stackrel{h}{0}=\stackrel{v=48}{48t}-16t^2\implies 0=16t(3-t^2)
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0=16t\implies 0=t\impliedby \textit{when it first takes off from the ground}
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0=3-t^2\implies t^2=3\implies t=\sqrt{3}\impliedby \textit{when it came back down}[/tex]
