Acar traveling at a speed of 30.0 m/s encounters an emergency and comes to a complete stop. how much time will it take for the car to stop if it decelerates at -4.0 m/s^2?

So for this one you have vf=0 because in the end the velocity would be 0, vo or velocity initial would be 30m/s because that's the initial speed we are looking at and you have -4.0 m/s^2 as your acceleration. You would use the formula vf=vo+at and plug those in to find your t.

So, it would be
 0=30-4(t)
-30=-4(t)
-30/-4=t
t=7.5 seconds
The answer would be 7.5 seconds.

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