二次方程 函数

二次方程 函数
The following function gives the height,h metres,of a batted baseball as a function of the time,t seconds,since the ball was hit:
h= -6(t-2.5)^2+ 38.5
a) How many seconds after it was hit did the ball hit the ground ,to the nearest second?
请列出主要的演算步骤和式子.