While sledding down a snowy hill Ed slowed down from 5 m/s to rest in a distance of 100 m. What was Ed's acceleration?
Since you also have time as an unknown value, you need 2 equations that combine these values. By using the equations of speed and distance for deceleration, the answer is:
This is the simple elementary path. If you are new to motion, you want to go this path.
Provided that the acceleration is constant, we know that:
Then substituting in
This path is not for beginners, as it is the calculus path. All it provides is actual proof of the above equations. I am just posting in case you are interested in how it works.