y=(1/3)x^3-x^2-3x+9
y‘ = x^2-2x-3 = (x+1)(x-3)
x＜-1时单调增；-1＜x＜3时单调减；x＞3时单调增
极大值f(-1) = -1/3-1+3+9 = 32/3
极小值f(3) = 9-9-9+9 = 0