Remember that in , h stands for the height. When the ball hits the ground, the height is 0, so plug that in for h. This gives:
Now, solve that by the quadratic equation to figure out what t equals when h is zero.
Similarly, you can find out the two times the ball was at a height of 17m by plugging in 17m for h:
Move the 17 to the other side, then solve by factoring / quadratic equation. You'll get two times. The ball was ABOVE 17m between those two times, so you can find the length of time simply by subtracting.
See how that goes!