Xavier is roller skating at 14 km/hband tosses a set of keys on the ground at 8 km/h the speed of the keys relative to the ground is

Still 14 km/h

Assuming Xavier through the keys straight down and not at an angle. The keys will travel along the ground at the same speed Xavier is rolling along the ground in the x direction.

Doesn't matter how hard he tosses the keys to the ground whether it was 8km/h or 100km/h, they still travel at 14 km/h relative to the ground in the x direction.