The schema is incorrect. It violates second normal form. MaKH does not belong in tb2_ChiTietHopDong. You would obtain the MaKH value by joining to tbl2_ThongTinHopDong.
You didn't explain what exactly your problem was but when you have two related tables in a query and you try to delete a row (you CANNOT delete a column, you only delete rows), it is the "child" table that you delete from NOT the parent table. If you want to delete a row from a parent table based on a value in the child table, you MUST have cascade delete specified since you obviously cannot delete a parent that has child record without it AND you need to use a sub query to identify the row since you can only delete child rows from a query with a join.
