var PrinterInfo = from pInfo in db.Printer
                                join pModel in db.PrinterModel on pInfo.FirmID equals pModel.FirmID

                                  select new
                                  {
                                      pInfo.SerialNumber,
                                      pInfo.Firm.NameProduct,
                                      pModel.ModelName
                                  };

                dgvPrinter.DataSource = PrinterInfo.ToList();