[填空题]
A bicyclist in the Tour de France crjf wt6x:xc9at4;a-r ou /rbnd7t2a;s ests a mountain v :hqvxts+ nz e,y767npass as he moves at 18 km/h . At the bottom, 4.0 km farther, his spez7 nvt7+ s:qx,h e6nyved is 75 km/h . What was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$
