Obtaining deceleration from speed, distance and time

Is it possible to obtain the rate of deceleration of an object, if you know it's original speed/sec, expected distance to travel, and expected time to take e.g.

Object A is travelling at 800 units per second, towards a point (Z) which is 400 units away. A is expected to reach this point in 1 second, as it is decelerating all the way (reaching 0 units/second upon reaching Z) at a constant rate.

I hope I've explained what I'm trying to do well enough.