Basically, the question is: how many seconds is h greater than 21? (This tennis ball is being thrown on a planet other than Earth, by the way. I challenge anyone to throw a tennis ball that stays in the air anywhere near as long as this one does.)
To figure it out, solve for the two times the equation equals 21. The first time will be when the ball crosses into view, and the second one will be the time the ball falls out of view. The time in between is the answer you want.
So there you go–the ball is at height 21 at 1 second and at 21 seconds. There are 20 seconds between there, so that’s the amount of time the ball is visible to the kids on the roof.
The other way some folks might choose to solve this is by graphing. If you graph the given function and also a horizontal line at y = 21, you get a nice visual of the ball’s flight, which helps make the 20-second window the ball is visible more intuitive.
from Tumblr https://ift.tt/2KKOGTC