Math Help (Arithmetic Series)?

The question says: "The sum of the first 12 terms in an arithmetic series is 186, and the 20th term is 83. What is the sum of the first 40 terms?"

The answer is 3420, but I can't figure out how to get there. Any help is appreciated.

The sum of the first 12 terms in an arithmetic series is 186, and the 20th term is 83.
What is the sum of the first 40 terms?
If a series is arithmetic the sum of the first n terms, denoted Sn,
there are ways to find its sum without actually adding all of the terms.
To find the sum of the first n terms of an arithmetic series use the formula,
Sn = n/2(a1 + an) where n is the number of terms,
a1 is the first term and an is the last term.
186 = 6(a1 + an)
a1 + an = 31
2a1 + 11d = 31
a1 + 19d = 83
2a1 + 38d = 166
27d = 135
d = 5
a1 = -12
S40
= 20(-24 + 195)
= 20(171)
= 3420