Suppose that I'm standing on the ground and throw a rock into the air as hard as I can. The function Ht = 6 + 60t - 10t ² models the rock's height above ground in feet after t seconds. For example, the model says H(0) - 6 because I am six feet tall, so the rock is six feet above the ground the moment I throw it. 

(a) When will the rock hit the ground again? 

(b) If the roof of Deady Hall is 100 feet above ground where I'm standing, am I able to throw the rock above the roof? Why or why not?