[填空题]
A bicyclist in the Tour de Fra(equc ut 20;ed3go(t /:czdlr nce crefqa-.bk unab,:gdo//9 6 sxj(qynlv2sts a mountain pass as he moves at 18 km/h . At the botto2n. /k9:qy fbxud ajn(ovg/ al,qbs-6m, 4.0 km farther, his speed is 75 km/h . What was his average acceleration (in $m/s^2$) while riding down the mountain? $m/s^2$
