The acceleration of an object is the rate of change of its velocity. Mathematically it is given as:

a = dv/dt

where a is the acceleration, V is velocity and t is the time. This can also be given as,

a = (v2 - v1)/t

where v2 and v1 are velocities at two different times and t is the time duration between instance 1 and 2.

In this case, the velocity of the roller coaster changes from 25 m/sec to 10 m/sec in a duration of 3 sec. Hence, the acceleration of the roller coaster can be calculated as:

a = (10 -25)/3 = -15/3 = -5 m/s^2

Since, the velocity is decreasing, the roller coaster is decelerating ( and hence the negative sign) at a rate of 5 m/s^2.

The roller coaster may again accelerate as it moves from top of the hill to bottom of the hill.

Hope this helps.