Calculate the change in momentum of a 0.5kg ball that strikes the floor at 15 m/s and bounces back up at 12 m/s

The change in momentum is:  

Explanation:

Let's define that vectors pointing up are positive, and vectors pointing down negative.

Then we express the initial momentum of the ball as the product of its mass times the velocity, and include the negative sign since this momentum is pointing down (velocity vector is pointing down):

the final momentum is positive (pointing up) and given by the product:

Therefore, since the change in momentum is defined as the difference between the final momentum minus the initial one, we get:

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