`7)`
`3x^2 - 4y + 4x - 3y^2`
`= 3x^2 - 3y^2 + 4x - 4y`
`= 3(x^2 - y^2) + 4(x - y)`
`= 3(x - y)(x + y) + 4(x - y)`
`= (x - y)[3(x + y) + 4]`
`8)`
`x^4 + 5x^3 + 15x - 9`
`= x^4 - 9 + 5x^3 + 15x`
`= (x^2 - 3)(x^2 + 3) + 5x(x^2 + 3)`
`= (x^2 + 3)(x^2 - 3 + 5x)`
`9)`
`x^4 - 6x^3 + 54x - 81`
`= x^4 - 81 - 6x^3 + 54x`
`= (x^2 - 9)(x^2 + 9) - 6x(x^2 - 9)`
`= (x^2 - 9)(x^2 + 9 - 6x)`
