Deriving/Applying the Geometric Series Formula Practice Test

Derive the finite geometric series formula starting from

$$S_n = a + ar + ar^2 + \cdots + ar^{n-1},$$

by multiplying by $r$, subtracting to cancel middle terms, and solving for $S_n$ (assume $r \ne 1$). Which final expression is correct?

For the geometric series $2 + 6 + 18 + 54$, identify $a$, $r$, and $n$, then find the sum using $S_n = \frac{a(1-r^n)}{1-r}$.