Answer
(a) In 8 seconds the object will strike the ground.
(b) The height of the object is 896 feet after 4 seconds.
Work Step by Step
(a) t must be found when s is 0, so:
$0=1280-32t-16t^2$
$0=16(80-2t-t^2)$
$0=80-2t-t^2$
Factor:
$0=(8-t)(10+t)$
Two solutions:
$0=(8-t_1)$
$t_1=8$
$0=(10+t_2)$
$t_2=-10$
Since time can't be negative, only the first answer is correct.
(b) The time is given, so the height (s) must be found:
$s=1280-32(4)-16(4)^2$
$s=1280-128-16(16)$
$s=1280-128-256$
$s=1280-128-256$
$s=896$
