How do you find the sum of the arithmetic series -12-9-6-...+39?
There are 2 formulae to do this, but you need to find how many terms there are first.
The sum of an Arithmetic series can be found from:
However both of these require knowing how many terms there are.
Each term is defined by
What do we know already?
Now we can use either formula for the sum of the first 18 terms.