Determine the pattern rule for the given data points (n, M): (1, 3), (2, 5), (4, 9), (7, 15), (10, 21).

2.4.

Step 1: Determine the pattern rule.
Let $n$ be the Pattern Number and $M$ be the Number of Matchsticks.
From the given data points $(n, M)$: $(1, 3), (2, 5), (4, 9), (7, 15), (10, 21)$.
We observe the differences in $M$ and $n$:
For $n=1$ to $n=2$: $\Delta n = 1$, $\Delta M = 2$. Ratio $\frac{\Delta M}{\Delta n} = \frac{2}{1} = 2$.
For $n=2$ to $n=4$: $\Delta n = 2$, $\