A 0.4 kg ball moves towards a wall at 7 m/s and bounces straight back at 5 m/s…

Step 1. (a) $0.4\ \text{kg} \times 7\ \text{m/s} = 2.8\ \text{kg m/s}$.

Step 2. (b) $0.4\ \text{kg} \times (-5) = -2\ \text{kg m/s}$.

Step 3. (c) $-2 - 2.8\ \text{kg m/s} = -4.8\ \text{kg m/s}$, pointing away from the wall.