`c ) ( 2x + 3y )^2 - ( 2x - 3y )^2 - 12xy`

`= 4x^2 + 12xy + 9y^2 - ( 4x^2 - 12xy + 9y^2 ) - 12xy`

`= 4x^2 + 12xy + 9y^2 - 4x^2 + 12xy - 9y^2 - 12xy`

`= ( 4x^2 - 4x^2 ) + ( 12xy + 12xy - 12xy ) + ( 9y^2 - 9y^2 )`

`= 12xy`

`d ) ( x - 5 ) ( -x^2 + x + 1 )`

`= -x^3 + x^2 + x + 5x^2 - 5x - 5`

`= -x^3 + ( x^2 + 5x^2 ) + ( x - 5x ) - 5`

`= -x^3 + 6x^2 - 4x - 5`

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