The expression is .

This is the sum of an arithmetic sequence , with first term .

and the common difference is d = 3.  the nth term is .

To find the value of n.

The nth term of an arithmetic sequence .

                 (Substitute , and d = 3)

               (Add 1 to each side)

                          (Divide each side by 3)

The sum of the first n terms of an arithmetic sequence .

The solution is .