The recurrence relation $a_n = 3a_{n-1} - 2a_{n-2} + c$ (where $c$ is a constant) has homogeneous solution $a_n^{(h)} = c_1 \cdot 2^n + c_2 \cdot 1^n$. To find the particular solution for the non-homogeneous term $c$, which form should $a_n^{(p)}$ take?