I got as far as

 var invoicesList = (from invoices in _dbSet.Where(x => x.Account == accountName && x.Hidden == false)
                                join transact in _siriusContext.Transact on invoices.InvoiceNo equals transact.InvoiceNo into InvoiceBalance
                                from rt in InvoiceBalance.DefaultIfEmpty()
                                select new InvoicesList
                                {
                                    InvoiceNo = invoices.InvoiceNo,
                                    Account = invoices.Account,
                                    Hidden = invoices.Hidden,
                                    Descrip = invoices.Descrip1,
                                    Created = invoices.DateTime,
                                    Finalized = invoices.Finalized,
                                    Balance = (decimal?)rt.Extension??0
                                });

but this is still not what I want as I want sum of extension. Still struggling.