Well you always need to know the 1st term to actually calculate the 5th and 6th. For the series may begin with 11 or any other number and the fifth and sixth could be different then. Wont call it useless info

You don't need to know the first term here to calculate the 5th and 6th term, though, because the formula given is for calculating the nth term. That would only be the case if it was something like, "here's an arithmetic sequence with common difference 3, what's the 5th term?" Any time they give you an explicit function for the nth term, you can just plug in your value of n.

I think that the first few terms of the sequence are included just for the sake of clarity. This is a fairly common practice, I think. I'm not sure what the exact GMAT policy is on useless information, but I bet we could find other examples of problems that include information that isn't strictly necessary but is added for the sake of clarity.