Recall that acceleration is defined as the change in velocity (from initial velocity vi to final velocity vf) over time (it is a rate) so in our case is going from 15 m/s to 10 m/s in a total ot 5 seconds.
Our acceleration then becomes:
a = (vf - vi) / t = (10 - 15) / 5 = -5 / 5 = -1 m/s^2
Notice the acceleration is negative ( the object is reducing its vekocity) and also the units are in meters per second SQUARED.
