>>>Off the top of my head, if I understood your problem correctly. It would be something like this:
>>>

>>>var list = (from  d in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
>>>               from t in transact .Sum(s=>s.extension).Where(w=>w.extension != null && w.invoiceNo == d.InvoiceNo).
>>>              select(x => new InvoicesList { InvoiceNo = x.InvoiceNo, Account = x.Account, Hidden = x.Hidden, 
>>>                                                          Descrip = x.Descrip1, Finalized = x.Finalized, Created = .DateTime}, InvoiceBalance= t)).ToList();
>>>

>>
>>I am trying to use this syntax, but getting errors and can not figure this out. Here is my attempt, but I don't see where to put Sum here:
>>
>>

>> var invoicesList =  (from  d in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
>>                 from i in _siriusContext.Transact.Where(w=>w.Extension != null && w.InvoiceNo == d.InvoiceNo)
>>                select(x => new InvoicesList { InvoiceNo = x.InvoiceNo, Account = x.Account, Hidden = x.Hidden, Descrip = x.Descrip1, Finalized = x.Finalized, Created = x.DateTime, Balance= i.Extension}))
>>

>
>My example had the sum in it... btw, what are the errors?

If I leave sum where you have it, then it says that decimal doesn't have Sum operator (something along the lines).

I found this sample http://stackoverflow.com/questions/1285710/linq-to-sql-join-and-sum which I'll try to adapt now.