A bouncy ball moves toward a wall at 10 m/s. Ignore friction. What is its final velocity?

-10 m/s (assuming no energy loss happens)

Explanation:

Velocity is a vector so it has a direction.

If you consider the forward direction (towards the wall) as positive then you should consider the backward direction (away from the wall) as negative.

Assuming no energy loss happens the ball should bounce back at the same speed as it bumped in but in opposite direction. So final velocity is - 10m/s