〔（x^2+18x+72)÷（x+12)-(x^2+5x-6)÷(x-1)〕·(x^3-2x^2-x+1)
= [(x+12)(x+6)/(x+12) - (x-1)(x+6)/(x-1)] ·(x^3-2x^2-x+1)
= [(x+6) - (x+6)] )·(x^3-2x^2-x+1)
= 0 ·(x^3-2x^2-x+1)
= 0