$ \begin{aligned} 解:原式&=6x^3+9x^2-12x-2x^2-3x+4 \\ &=6x^3+7x^2-15x+4. \\ \end{aligned}$ $ $ $ $ $ $ $ \begin{aligned} 解:原式&=x^3-x^2y+xy^2+x^2y-xy^2+y^3 \\ &=x^3+y^3. \\ \end{aligned}$ $ $ $ \begin{aligned} 解:原式&=-2x^5+3x^4-x^3+4x^4-6x^3+2x^2-10x^2+15x-5 \\ &=-2x^5+7x^4-7x^3-8x^2+15x-5. \\ \end{aligned}$ $ $ $ \begin{aligned} 解:原式&=2x²-x-1-2x²+6x+20 \\ &=5x+19. \\ \end{aligned}$ 解:原式=6a²-9a+2a-3-4a²+16a+5a-20=2a²+14a-23, 当a=-2时,原式=2×(-2)²+14×(-2)-23=-43. 解:原式=3x²-xy-6xy+2y²-3x²+6xy+12x=-xy+2y²+12x， 当x=1,y=-2时,原式=-1×(-2)+2×(-2)²+12×1=22. 解:根据题意,得(x+1)(3x-4)=3x(x-3)+8, 去括号,得3x²-4x+3x-4=3x²-9x+8, 移项、合并同类项,得8x=12, 系数化为1,得x=1.5, 故当x=1.5时,M=N.
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