Question 847199
I'm not sure I understand your question.

The sum of terms between 100-600 would be 175350 if you took  sum (1 to 600) - sum(1 to 100)  =  (600)(601)/2 -  (100)(1010/2.

Where you lose me is how can the first term be 8 if we're talking about numbers between 100-600? 

The common difference just means that I am to increment by 6 each time.  For instance:  1,7,13,19 all have common difference 6. 

I hope I have given you enough information to help.  If not, please send me an email at swincher4391@yahoo.com so we can discuss more thoroughly.