(x³-2x²+x-4)-(2x³-5x-4)去括号
(x³-2x²+x-4)-(2x³-5x-4)去括号
(x³-2x²+x-4)-(2x³-5x-4)=x³-2x²+x-4-2x³+5x+4
=-x³-2x²+6x