6s返回,就是位移=0
0=9*6-0.5*a*6^2
a=3m/s^2
12=9t-0.5*3*t^2
t=4s
v=9-3*4=-3m/s
(2)x1=7.5m
x3=13.5m
x5=13.5
x7=-13.5