`a)`
`A+B-C`
`=(x^2-2x-y^2+3y-1)+(-2x^2+3y^2-5x+y+3)-(3x^2-2xy+7y^2-3x-5y-6)`
`=x^2-2x-y^2+3y-1-2x^2+3y^2-5x+y+3-3x^2+2xy-7y^2+3x+5y+6`
`=(x^2-2x^2-3x^2)+(-2x-5x+3x)+(-y^2+3y^2-7y^2)+2xy+(6-1+3)+(3y+y+5y)`
`=-4x^2-4x-5y^2+2xy+9y+8`
`b)`
`A-B+C`
`=(x^2-2x-y^2+3y-1)-(-2x^2+3y^2-5x+y+3)+(3x^2-2xy+7y^2-3x-5y-6)`
`=x^2-2x-y^2+3y-1+2x^2-3y^2+5x-y-3+3x^2-2xy+7y^2-3x-5y-6`
`=(x^2+2x^2+3x^2)+(-y^2-3y^2+7y^2)+(-2x+5x-3x)+(3y-y-5y)-(1+6+3)-2xy`
`=6x^2+3y^2-3y-10-2xy`
`c)`
`-A+B+C`
`=-(x^2-2x-y^2+3y-1)+(-2x^2+3y^2-5x+y+3)+(3x^2-2xy+7y^2-3x-5y-6)`
`=-x^2+2x+y^2-3y+1-2x^2+3y^2-5x+y+3+3x^2-2xy+7y^2-3x-5y-6`
`=(-x^2-2x^2+3x^2)+(y^2+3y^2+7y^2)+(2x-5x-3x)+(1+3-6)-2xy+(-3y+y-5y)`
`=11y^2-6x-2-2xy-7y`