(6-2ab-18^2b3)/12a⁴b
Perhaps you mean:
$$\\\frac{(6-2ab-18^2b^3)}{12}\times a^4b\\\\ =\frac{(3-ab-9^2b^3)}{6}\times a^4b\\\\ =\frac{a^4b(3-ab-81b^3)}{6}\\\\$$
Now, if you want to, you could expand the brackets.