Answer:
It takes 4 seconds for the projectile to hit the ground
Explanation:
The height of the projectile after t seconds is given by the following equation:

How long will it take the projectile to hit the ground?
It happens when

So


Solving a quadratic equation:
Given a second order polynomial expressed by the following equation:
.
This polynomial has roots
such that
, given by the following formulas:



In this question:

So




Time is a positive measure, so:
It takes 4 seconds for the projectile to hit the ground