Eyjafjallajokull Projectile

The height H, in feet, of out ash projectile
is given by the equation: H=-16t2^2 368t, where t is the time, in seconds.
We will assume that the volcano has no height.

H=0 at t=0
When does the ash projectile reach its maximum
height? What is its maximum height? When does the ash projectile return to the
ground?