When the ball reaches the ground, the height, h(t), is 0. Might as well make the numbers easier to work with - since for the x-intercepts (t-intercepts), h(t) = 0, you can divide the entire equation by the greatest common factor, in this case -16, to make things better:
Now you can solve this any way you like, such as by using:
This is an easy polynomial though, and you may be able to factor it just by eyeballing it:
You can't have negative time, so we reject -1, leaving only 9 as the answer.