How many carbon atoms are in the longest chain?

 The answer is 9. The \ / and | are my best representations of the c-c bonds in yahoo. 
Start your numbering from the long chain neartest the bottom of the page and it goes like this: 
1 is very start( the furthest down carbon) 
2 is the only carbon connected to the first carbon \ 
3 is the only carbon connected to the second carbon | 
4 is the only carbon connected to the third carbon \ 
5 is the carbon chain pointing up | 

Then start going right. 
6 is the carbon to the right of 5 / 
7 is the carbon to the right of 6 \ 
8 is the carbon to the right of 7 / 
9 can be either of the 2 single lines coming off of 8( it doesn't matter which one because each will result in a carbon bond with a methyl bond off of it) 

That is all there is too it. My guess is that this is what you did when you got 8 but you forgot to count the very first carbon. I hope this helps.