smc

If $5b9$ is divisible by $9$, this must mean that $5+b+9$ is a multiple of $9$. So, $$5+b+9=9,18,27,\mathrm{36...}$$ Because $5+9=14$ and $b$ is in between 0 and 9, $$5+b+9=18$$ $$b=4$$ The question states that $$2a3+326=549$$ so $$2a3=549-326$$ $$2a3=223$$ $$a=2$$ $$a+b=6$$ which is answer choice $\boxed{6}$.