3x^2-5x-7+6x^2-5x-2
=3x^2+6x^2-5x-5x-2-7
=9x^2-10x-9再问下，差等于呢？3x^2-5x-7-(6x^2-5x-2)=3x^2-5x-7-6x^2+5x+2=3x^2-6x^2-5x+5x+2-7=-3x^2-5