A rock is projected directly upward from ground level, and its height as a function of time is h(t)=-60t^2+90t.
a) make a sketch of h(t)
b) Determine the time at which the rock reaches its maximum height, and find that maximum height in feet.
c) For what is the interval will the rock be more than 120 feet above the ground?
d) After how many seconds will the rock hit the ground?