Question 43505
Since we want the object to hit the ground, the height will be zero.
{{{s(t) = -16t^2 + 64t + 80}}}
{{{0 = -16t^2 + 64t + 80}}}
{{{0 = -2t^2 + 8t + 10}}}
{{{0 = -2t^2 + 10t - 2t + 10}}}
{{{0 = (-2t^2 + 10t) + (-2t + 10)}}}
{{{0 = -2t(t - 5) - 2(t - 5)}}}
{{{0 = (-2t - 2)(t - 5)}}}
{{{t = 5}}} and {{{t = -1}}}