If an object is moving with a velocity of 24m/s and has an acceleration of - 4m/s how long will it take to stop

The negative sign on the acceleration is only a vector quantity that means the object is accelerating to the left. Hence, we can only focus on it magnitude which is 4 m/s^2. Acceleration is the change in velocity over time. The change in velocity must be 24 m/s - 0 m/s, if you want the object to stop. Therefore,

a = (v2 - v1) / t

4 = (24 - 0) t

t = 6 seconds

The object will stop after 6 seconds.