Verified answer

[tex] - \frac{1}{3} {x}^{2} - x + 6 = - \frac{1}{3} ( {x}^{2} + 3x - 18) = \\ - \frac{1}{3} ( {x}^{2} + 6x - 3x - 18) = \\ - \frac{1}{3} (x(x + 6) - 3(x + 6)) = \\ - \frac{1}{3} (x + 6)(x - 3)[/tex]