How do you measure average acceleration?

I'm doing this project for Algebra 2, and I need to find the average acceleration for a roller coaster. It says to use the formula S=(Vo)t+1/2at^2. T= 135.27 seconds. S(distance)= 1488 meters. Additionally, it is 11M per second. I keep getting a small decimal answer, I'm not sure why, it should be 5 or somewhere in that ballpark. Thanks!