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)