After 9 seconds.
(t) = -16t^2 + 1296 // 1296 because that’s the initial height
Now find what value of t makes h(t) equal to 0.
h(t) = 1296 - 16t^2
Use the difference of squares formula
h(t) = (36-4t)(36+4t)
We only take the positive answer, not the negative one (because negative won't make sense here)
36 - 4t = 0
36 = 4t
9 = t
