3x²y-8x³-5xy²-(2x³-3y³)
=3x²y-8x³-5xy²-2x³+3y³
=3x²y-10x³-5xy²+3y³
所以
3x²y-8x³-5xy²-(3x²y-10x³-5xy²+3y³)=2x³-3y³,括号里面是填的代数式