How long did it take to reach the bottom of the slope

To solve this we are going to use the kinematic equation for final velocity: 
where
is the final velocity
is the initial velocity 
is the acceleration 
is the time

We know for our problem that the the velocity at the beginning of the slope is  , so . We also know that the speed ad the bottom of the slope is , so . We also know that the acceleration is , so . Lets replace those values in our formula and solve for :




seconds

We can conclude that it takes the roller coaster 3 seconds to reach the bottom of the slope.

Whose mistake? can you give more details to what the question is asking?