#### Answer

6.5 s

#### Work Step by Step

We know that:
$$ 0 = -16t^2 +20t+550$$
Note, we subtract 550 since we want to find when the object is 550 feet off of the ground. Next, we find:
$$x= \frac{-b \pm \sqrt{b^2-4ac}}{2a}$$
Where b is the coefficient of the term with a variable raised to the first power, a is the coefficient of the squared term, and c is the coefficient of the term without a variable in front of it.
Plugging in the given values and solving, we get:
6.5 s