f(x)=x³+x²-(a+2)x²-(a+2)x+2ax+2a
=x²(x+1)-(a+2)x(x+1)+2a(x+1)
=(x+1)[x²-(a+2)x+2a]
=(x+1)(x-a)(x-2)
所以零点是-1,a,2