3x^4+6x^3-12x^2-15x-18
=3(x^4+2x³-4x²-5x-6)
=3(x^4+2x³-3x²-x²-5x-6)
=3[x²(x^2+2x-3)-(x²+5x+6)]
=3[x²(x+3)(x-1)-(x+3)(x+2)]
=3(x+3)(x³-x²-x-2)
3x^4+6x^3-12x^2-15x-18
=3(x^4+2x³-4x²-5x-6)
=3(x^4+2x³-3x²-x²-5x-6)
=3[x²(x+3)(x-1)-(x+3)(x+2)]
=3(x+3)(x³-x²-x-2)
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