Determine the pattern rule for the given sequence of matchsticks.

2.4.

Step 1: Determine the pattern rule.
The differences in the number of matchsticks for corresponding differences in pattern numbers are constant. For example, from $n=1$ to $n=2$, $\Delta n = 1$ and $\Delta M = 5-3=2$. From $n=2$ to $n=4$, $\Delta n = 2$ and $\Delta M = 9-5=4$. The ratio $\frac{\Delta M}{\Delta n} = \frac{2}{1} = \frac{4}{2} = 2$.
This indicates a linear relationship of the form $M = 2n + c$.
Using the point $(1, 3)$ from the table:

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