The heght above ground level in meters of a missile lauched veryically is given by h(t)=-16t3+100t
at what time is the missile 72 m above ground level t is time in second
It is NOT !!!
h(t) = -16 t^2 + 100 t [ NOT -16 t^3 !!! ]
where 100 is initial feet per second up and -16 is half of g = 32 ft/second^2 in those obsolete units.
anyway I will assume you mean 72 FEET
72 = -16 t^2 + 100 t
16 t^2 -100 t + 72 = 0
solve that quadratic
t = 0.83 seconds on the way up and 5.42 seconds on the way back down.