`a)`
`A(x)=6x^3-3x^2+4x^4-2-7x`
`=4x^4+6x^3-3x^2-7x-2`
``
`B(x)=3x^2+5x-8-6x^3-4x^4`
`=-4x^4-6x^3+3x^2+5x-8`
``
`b)`
`M(x)=A(x)+B(x)`
`=(4x^4+6x^3-3x^2-7x-2)+(-4x^4-6x^3+3x^2+5x-8)`
`=4x^4+6x^3-3x^2-7x-2-4x^4-6x^3+3x^2+5x-8`
`=(4x^4-4x^3)+(6x^3-6x^3)+(-3x^3+3x^2)+(-7x+5x)+(-4-8)`
`=-2x-12`
``
Để `M(x)` có nghiệm
`\to M(x)=0`
`\to -2x-12=0`
`\to -2x=12`
`\to x=-6`
Vậy `x=-6`
``
`c)`
`N(x)=A(x)-B(x)`
`=(4x^4+6x^3-3x^2-7x-2)-(-4x^4-6x^3+3x^2+5x-8)`
`=4x^4+6x^3-3x^2-7x-2+4x^4+6x^3-3x^2-5x+8`
`=(4x^4+4x^4)+(6x^3+6x^3)+(-3x^2-3x^2)+(-7x-5x)+(-2+8)`
`=8x^4+12x^3-6x^2-12x+6`