Adam is riding forward on an electric scooter at 10 m/s. He throws a ball to Robert, who is stationary. The ball travels at a speed of 8 m/s relative to Adam. How fast is the ball moving relative to Robert as he tries to catch it?

Where are you getting stuck? Do you at least have a guess as to what the answer is?

Nope

Adam is moving at 10 m/s, and the ball goes an additional 8 m/s faster than that. So how fast is it traveling in all?

I guess 18

You got it. That's all there is to it. :)

OK