13. \( \frac{(-6 a)(-2 a)(-a)}{-6 a-2 a-a} \)

First, let's simplify the expression step by step. The numerator is \((-6a)(-2a)(-a)\), which results in \(-12a^3\) (note that the product of the negatives gives a negative sign). Next, we simplify the denominator: \(-6a - 2a - a\). Adding these together gives \(-9a\). Now, we have: \[ \frac{-12a^3}{-9a} \] The negatives cancel out, and we can simplify this to: \[ \frac{12a^3}{9a} = \frac{12}{9} \cdot a^{3-1} = \frac{4}{3} a^{2} \] So, the final simplified result is: \[ \frac{4}{3} a^{2} \]