Write the sum using sigma notation.
3 – 6 + 9 – 12 + ...
We have alternating terms, so we need a factor of (-1) raised to some power.
The general term
an = (-1)n + 13n
makes sense if we start at n = 1. Then the sum looks like
There are other correct answers. For example, we could change the exponent on the (-1) and write