`a, 3x - 3y + x^2 - y^2`
`= (3x - 3y) + (x^2 - y^2)`
`= 3 . (x - y) + (x - y)(x + y)`
`= (x - y) . (3 + x + y)`
`b, x^2 - 4x^2y^2 + y^2 + 2xy`
`= (x^2 + 2xy + y^2) - 4x^2y^2`
`= (x + y)^2 - 4x^2y^2`
`= (x + y)^2 - (2xy)^2`
`= [(x + y) - 2xy] . [(x + y) + 2xy]`
`= (x + y - 2xy) (x + y + 2xy)`
`c, x^6 - x^4 + 2x^3 + 2x^2`
`= (x^6 - x^4) + (2x^3 + 2x^2)`
`= x^2 . (x^3 - x^2) + 2 . (x^3 + x^2)`
`= - [x^2 . (x^3 + x^2)] + 2 . (x^3 + x^2)`
`= - [(x^3 + x^2) . (x^2 + 2)]`
`d, x^3 - 3x^2 + 3x - 1 - y^3`
`= (x^3 - 3x^2 + 3x - 1^3) - y^3`
`= (x - 1)^3 - y^3`
`= (x - 1 - y) . (x - 1 + y)`
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