Answer;
3 seconds
Explanation;
Acceleration is the rate of change in linear velocity.
Acceleration = (final velocity -initial velocity) /time
In this case;
Final velocity = 0 m/s (because the car is going to rest by stopping)
Initial velocity = 24 m/s
Acceleration = -8.0 m/s²
Thus; - 8.0 m/s² = (0-24 m/s)/ t
time = (-24 m/s)/-8.0 m/s²
= 3 seconds