[填空题]
A bicyclist in the Tour d( ubq)1l4xb wse France crests a mountain pass as he moves at 18 km/azgt h q:+:0hsh . At the bottom, 4.0 km farther, his speed is: h:zaqtg hs+0 75 km/h . What was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$
