smc

Setting $y=0$ we have that the $x-$intercept of the line is $x=\frac{6}{a}$. Similarly setting $x=0$ we find the $y-$intercept to be $y=\frac{6}{b}$. Then $\frac{18}{ab}=\frac{1}{2}\cdot \frac{6}{a}\cdot \frac{6}{b}$ so that $\frac{18}{ab}=6$, simplifying we would get $ab=3$. Hence the answer is $\boxed{3}$.