m=x³-3x²y+2xy²+3y³ ①
n=x³-2x²y+xy²-5y³ ②
①×3得3m=3x³-9x²y+6xy²+9y³ ③
③-②得3m-n=2x³-7x²y+5xy²+14y³
即2x³-7x²y+5xy²+14y³=3m-n