What is an object's acceleration if it is moving at 30 m/s and comes to a stop in 5 s?

Using the formula:

a = (Vf - Vi) / t

The object is coming to a stop, so the initial velocity will be 30 m/s and the final velocity will be 0 m/s. The time is 5s, as stated in the problem.

a = (0 - 30) / 5

a = - 6 m/s^2

(Also, the negative sign is important because it shows a deceleration)