Known 3, - 6,9,12 Complete the following questions: find the sum of this column?

Known 3, - 6,9,12 Complete the following questions: find the sum of this column?

I'm sorry, your title is wrong
It should be three, six, nine, twelve ,-2004,2007,-2010
Because a1 + A2 = - 3
A3+A4=-3
...
A669+A670=-3
therefore
S=-3*(670/2)=-1005