[填空题]
A bicyclist in the Tour jk erz w.ca-p:au)+ 7rde France crests ua le 38d1e7c,zi mhd7a mountain pass as he moves at 18 km/h . At the bottom, 4.0 km farther, his spemeuci , d773 zadlh18eed is 75 km/h . What was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$