the height above ground level in metres of missile luanched verticaly ,given by h(t)=-16tcube plus 100 t,what is missile 72m above ground level?
fixing your typos,
h(t) = -16t^2+100t
so now just solve
-16t^2+100t = 72
in the usual way.
I think you will also find that the height is in feet, not meters.