`a)`
`A-B=(6x^2-7xy-4y^2)-(-2x^2+7xy+5y^2)`
`=6x^2-7xy-4y^2+2x^2-7xy-5y^2`
`=(6x^2+2x^2)-(7xy+7xy)-(4y^2+5y^2)`
`=8x^2-14xy-9y^2`
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`b)`
`Q-(3x^4-2xyz)=xy+3x^4-5xyz-713`
`Q=(xy+3x^4-5xyz-713)+(3x^4-2xyz)`
`Q=xy+3x^4-5xyz-713+3x^4-2xyz`
`Q=xy+6x^4-7xyz-713`