For $a_n = 5a_{n-1} - 6a_{n-2} + 2 \cdot 3^n$ with $a_0 = 0, a_1 = 1$, the homogeneous characteristic roots are $r_1 = 3$ and $r_2 = 2$. Since $3$ is a characteristic root, what is the appropriate form for the particular solution?