To solve this problem you must apply the proccedure shown below:
 1- You have the following information give in the problem above:
 - The ball is thrown downward from the top of a 190​-foot building with an initial velocity of 21 feet per second.
- The height of the ball h after t seconds is given by this equation: h=-16t²-21t+190.
 2- h=0 when the ball strikes the ground, then you have:
 -16t²-21t+190=0
 3. When you solve the quadratic equation shown above, you obtain:
 t=2.85 seconds
 The answer is: 2.85 seconds.