Answer:
a = (v2 - v1) / t
For constant acceleration (positive) v2 (final velocity) must be greater than v1 (initial velocity)
Note that velocity is a vector quantity in this equation
For instance in the case of a freely falling body one might have v1 as 9.8 m/s and v2 as 29.4 m/s in 3 sec giving
a = (-29.4 - (-9.8)) / 3 = -19.6 m/s^2 where the positive direction was chosen as upwards and the acceleration is negative or downwards
