I'm still struggling with basic summation evaluation.

I've managed to get a formula like this:

$\displaystyle \sum_{k=2}^n \sum_{j=1}^{3n} 1$

But the 3n is confusing me and I'm not sure what my next step should be.

I think I evaluate down to :

$\displaystyle \sum_{k=2}^n(3n-j)$? and then where?

Thanks in advance.