We stop with 6 because
Edit: To address your other points:
Only if the grouping is necessary to clearly separate each term in the summation.1) Individual terms will be grouped by parentheses.
No, not necessarily. In problems like the one given, yes, will affect the number of terms in each element of the sequence.The n number will determine the amount of terms.
Well, that is the idea. This is how the definition is supposed to be interpreted.2) Sub in the number whereever you see it in the equation.
I am not sure what you mean by this. When evaluating the term, you should use the full given form.3) Always include the last term in the sumation if it differs from the form of the rest of the terms.