smc

Using the definition of $n_a!$, the quotient can be rewritten as $$\frac{72\cdot 64\cdot 56\cdots 8}{18\cdot 16\cdot 14\cdots 2}$$ Note that for a given integer $x$, $\frac{72-8x}{18-2x}=4$. Since $0\le x\le 8$, the quotient simplifies to $\boxed{4^9}$.