solving equations by isolating variables and the square root principle

Question

While riding in a hot-air balloon Sam accidentally dropped his phone .

The height (in meters) of his phone at any given time (t in seconds) is h(t)=−4.9t2+400.

How long will it take for the phone to crash to the ground?

Show/explain your process.

Answer 1

 h(t)=−4.9t²+400

Sam dropped his at height h(t). When the phone touches the ground h(t) equals zero. So you need to solve

0=−4.9t²+400 for t

4.9t²=400

t²=400/4.9

t=20√/4.9 (we take only the positive root!)