Are you familiar with the summation notation? This might help: http://www.youtube.com/watch?v=hEPk36Yncxg

In long hand the expression is

(3(1) - 1) + (3(2) - 1) + (3(3) - 1) + (3(4) - 1) + .... + (3(n) - 1)

= (3 - 1) + (6 - 1) + (9 - 1) + (12 - 1) + .... + (3n - 1)

= 2 + 5 + 8 + 11 + .... + 3n - 1