-3x²+mx+nx²-x+3
=(-3x²+nx²)+(mx-x)+3
=(n-3)x²+(m-1)x+3
多項(xiàng)式的值與x無(wú)關(guān),說(shuō)明凡是含有字母x的項(xiàng)的系數(shù)都等于0
n-3=0 且 m-1=0
n=3 ,m=1
3m²+n-2n+5-2m²+7n+6
=(3m²-2m²)+(n-2n+7n)+(5+6)
=m²+6n+11
=1²+6×3+11
=1+18+11
=30