`a)`
`P(x)=8x^5+7x-6x^2-3x^5+2x^2+15`
`=(8x^5-3x^5)+(-6x^2+2x^2)+7x+15`
`=5x^5-4x^2+7x+15`
``
`Q(x)=4x^5+3x-2x^2+x^5-2x^2+8`
`=(4x^5+x^5)+(-2x^2-2x^2)+3x+8`
`=5x^5-4x^2+3x+8`
``
`b)`
`P(x)-Q(x)`
`=(5x^5-4x^2+7x+15)-(5x^5-4x^2+3x+8)`
`=5x^5-4x^2+7x+15-5x^5+4x^2-3x-8`
`=(5x^5-5x^5)+(4x^2-4x^2)+(7x-3x)+(15-8)`
`=4x+7`
Để `P(x)-Q(x)=4x+7` có nghiệm
`\to 4x+7=0`
`\to 4x=-7`
`\to x=-7/4`
Vậy `x=-7/4`