Find the sum of the first 15 terms of 2 + 6 + 18 + 54 + ...

S_n = n (first term + a_n) / 2

n = 15

S_15 = 15 (2 + a_n) / 2

We need a_n.

a_n = first term + d (n - 1)

a_n = 2 + d (15 - 1)

1. Find d.

2. Plug d into expression to find a_n.

3. After finding a_n, plug back into

S_15 = 15 (2 + a_n) / 2 to find the sum of the first 15 terms.

Take it from here.