s=-6(t^2- 15/6t)=-6(t- 15/6)^2+75/8，

When t = 15/6 and smax = 75/8m,

After the car braked, it stopped and advanced 75/8m.

S=V0t- 1/2at^2

V 0 =15m/sec A = 6 * 2 =12m/s 2

T = v0/a =15/12 =1.25 seconds.

s= 15* 1.25-6*( 1.25)^2=9.375m

This topic mainly investigates the problem of finding the maximum value by collocation method.