f'(x)=3x^2-6x-9
=3(x^2-2x-3)
=3(x-3)(x+1)
x=-1和3是f(x)的极值点
极值为f(-1)和f(3)
在[-2,-1]上增,[-1,2]上减.
f(x)max=f(-1)
f(-2)与f(2)中小的为f(x)的最小值