a,\(=x^4-3x^3+3x^3-9x^2-4x^2+12x-12x+36\)
\(=x^3\left(x-3\right)+3x^2\left(x-3\right)-4x\left(x-3\right)-12\left(x-3\right)\)
\(=\left(x-3\right)\left(x^3+3x^2-4x-12\right)\)
\(=\left(x-3\right)[x^2\left(x+3\right)-4\left(x+3\right)]\)
\(=\left(x^2-9\right)\left(x^2-4\right)\)