$\begin{array}{l}a)\,\,x^2 - xy - 6y^2\\ = x^2 - 4y^2 - xy - 2y^2\\ = (x-2y)(x+2y) - y(x+ 2y)\\ = (x+2y)(x - 2y - y)\\ = (x+2y)(x-3y)\\ b)\,\,x^3 - 3x^2 - 6x +8\\ = x^3 + 8 - (3x^2 + 6x)\\ = (x+2)(x^2 - 2x + 4) - 3x(x + 2)\\ = (x+2)(x^2 - 5x + 4)\\ = (x+2)(x-1)(x-4)\\ c)\,\,x^2 - 2xy + x + 3xz - 2y + 3z\\ = (x^2 + x) - (2xy + 2y) + (3xz + 3z)\\ = x(x+1) - 2y(x+1) + 3z(x+1)\\ = (x+1)(x-2y+3z)\\ d)\,\,(x-7)(x-5)(x-4)(x-2) - 72\\ = [(x-7)(x-2)][(x-5)(x-4)] - 72\\ = (x^2 - 9x + 14)(x^2 - 9x + 20) - 72\\ Đặt\,\,x^2 - 9x + 14 = t\quad ta\,\,được:\\ t(t + 6) - 72\\ = t^2 + 6t - 72\\ = t^2 + 12t - 6t - 72\\ = t(t+12) - 6(t+12)\\ = (t+12)(t-6)\\ = (x^2 - 9x + 26)(x^2 - 9x + 8)\\ = (x^2 - 9x + 26)(x-1)(x-8) \end{array}$
