To find the deceleration of the object. We need to know what is the speed the object is slowing down to, in this case it would be final velocity of 0.
Vi = 75
Vf = vi + at
0 = 300a + 75
-75 m/s = 300a
-75/300 = a
-0.25 m/s^2 = a.
The train slows down at this rate a = -0.25 m/s^2.
5 min = 5 • 60 = 300 seconds