the brakes of a car provide an acceleration of -5.0 m/s^2 . how long will it take for a car traveling at 20.0 m/s to stop ?

Answer 1

4-seconds.

But if you need to see the math, here you go:

Vf=Vi+at

Where

Vf=0m/s
Vi=20m/s
a=-5.0m/s
and
t=??

Solve for t:

0m/s=20m/s+(-5.0m/s^2)*t

-20m/s=(-5.0m/s^2)*t

t=-20m/s/-5.0m/s^2

t=4s