Can you show me the answer to this question and how you got your answer?

Question

A ball is thrown vertically upward from the ground. It’s distance in feet from the ground in t seconds is s=-16t^2+240t. After how many seconds will the ball be 864 feet from the ground?

Answer 1

864 = -16t^2 + 240t

16t^2 -240t + 864 = 0

t^2 - 15t + 54 = 0

(t-6)(t-9) = 0

t = 6 and 9

after 6 seconds it's at 864 feet

and again coming back down at 9 seconds it's at 864 feet

at 7.5 seconds it reaches maximum 900 feet, halfway between 6 and 9 seconds. In 2(7.5) = 15 seconds the ball hits the ground.