f'(x)=12x^2-6x-6=0
2x^2-x-1=0
(2x+1)(x-1)=0
极值点：x1=1,x2=-1/2
f(1)=4-3-6+2=-3
f(-1/2)=-1/2-3/4+3+2
=(20-5)/4=15/4