check the picture below.
thus is at 0 = -16t² + 96t +640,
[tex]\bf 0=-16t^2+96t+640\implies 0=-16(t^2-6t-40)
\\\\\\
0=t^2-6t-40\implies 0=(t-10)(t+4)\implies t=
\begin{cases}
\boxed{10}\\
-4
\end{cases}[/tex]
well, clearly it can't be a negative value for the elapsed seconds, so it can't be -4.