Final answer:
The speed of the tennis ball as observed by someone on the sidewalk is 8 km/h, which is the difference between Alana's skateboarding speed of 19 km/h and the ball's thrown speed of 11 km/h in the opposite direction.
Step-by-step explanation:
The speed of the tennis ball if someone was watching the exchange on the sidewalk would be the relative velocity of the ball with respect to the ground. Since Alana is moving at 19 km/h and throws the ball backward at 11 km/h to her friend, the speed of the ball as observed by someone on the sidewalk would be the difference in speeds. So, the speed of the ball concerning the sidewalk is 8 km/h (19 km/h - 11 km/h).