3.81: Product of binomial expansions: coefficients $a$ and $b$

The first three terms, in ascending powers of $x$, in the expansion of $\left(1 - \dfrac{x}{9}\right)^{18}\left(1 + \dfrac{x}{3}\right)$ are written in the form $1 + ax + bx^{2}$, where $a$ and $b$ are constants. Find the exact values of $a$ and $b$.