Abicyclist in the tour de france crests a mountain pass as he moves at 18 km/h. at the bottom, 3.8 km farther, his speed is 85 km/h. what was his average acceleration (in m/s2) while riding down the mountain?

Initial speed, u = 18 km/h
Final speed, v = 85 km,/h
Distance traveled, s = 3.8 km

Note:
1 km/h = 0.27778 m/s
Therefore
u = 18*0.27778 = 5 m/s
v = 85*0.27778 = 23.61 m/s
s = 3.8 km = 3800 m

Let a =  the acceleration.

Use the formula 
v² = u² + 2as, or
a = (v² - u²)/2s
   = (557.493 - 25)/7600
   = 0.07 m/s²

  0.07 m/s²

Gravity make the the ball go to the ground so the friction between the ground and the ball makes it bounce.