Answer:
[tex]2[/tex]
Step-by-step explanation:
[tex]h(t) = - 16 {t}^{2} + 64 \\ \Leftrightarrow 0 = - 16 {t}^{2} + 64 \\ \Leftrightarrow {t}^{2} = 4\Leftrightarrow t = \pm2\Leftrightarrow t = 2 \: (t \geqslant 0)[/tex]
