A = x4 + 3x3 - 9x - 9
A = x4 - 9 + 3x3 - 9x
A = (x2 - 3)(x2 + 3) + 3x( x2 - 3)
A = (x2 - 3)( x2 + 3 + 3x)
\(x^4+3x^3-9x-9\)
\(=x^4-9+3x^3-9x\)
\(=\left(x^2-3\right)\left(x^2+3\right)+3x\left(x^2-3\right)\) \(=\left(x^2-3\right)\left(x^2+3+3x\right)\)