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.