A moving trian with a velocity of 51 m/s east undergoes an acceleration of 2.3 m/s as it approaches a town. What is the velocity of the train 5.2 safter it has begun to decelerate?

I think you mean that 2.3 is deceleration and you want velocity after 5.2 seconds

If velocity is u and acceleration is a the velocity after time t is v, where

v = u + at

In your case, u = 51, a = -2.3 and t = 5.2 seconds

Substitute and get v