[填空题]
A bicyclist in the Tour de France crests aco0fw4due2y+rxhc.t8 q s*ih(;m m.g mountain pass as he moves at .r9h3lqqs8 2m eer 5ly,rw ngviq/715fqtox/18 km/h . At the bottom, 4.0 km farther, his s5l/tmq257nho giq,yvelq/ 1r srw q.fx re39 8peed is 75 km/h . What was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$
