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