①3m²n+5mn²+6nm²-4n²m-mn²=(3+6)m²n+(5-4-1)mn²=9m²n②3(a+b)²-(a-b)+2(a+b)²+4(a-b)-(a+b)²=(3+2-1)(a+b)²+(4-1)(a-b)=4(a+b)²+3(a-b)