（-x+2x^2+5)+(4x^2-3-6x)
=-x+2x^2+5+4x^2-3-6x
=6x^2-7x+2
（3a^2-ab+7)-(-4ab^2+2ab+7)
=3a^2-ab+7+4ab^2-2ab-7
=3a^2-3ab+4ab^2