[填空题]
A bicyclist in the Tourhpr yye4l8,tv)(. q6 ukgvrsix8 /2 jym; a-tkto9b2a0 2tq 8td76jdmd*lgtw de France crests a mountain pass as he moves at 18 km/h . At the bottom, 4.0 km farther, his speed is 75 km/h . Wht;d td t9qkmt27l *2-ba60wad8 jotm gat was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$
