check the picture below.
[tex]\bf h=-16t^2+56t\implies \stackrel{h}{0}=8t(-2t+7) \\\\[-0.35em] ~\dotfill\\\\ 0=8t\implies 0=t\impliedby \textit{when the ball took off from the ground} \\\\[-0.35em] ~\dotfill\\\\ 0=-2t+7\implies 2t=7\implies t=\cfrac{7}{2}\implies t=3\frac{1}{2}\impliedby \textit{down at }3\frac{1}{2}~seconds[/tex]
