C# .NET - I have take one combobox in datagridview coloumn and How to fetch its related database val

I have take one combobox in datagridview coloumn and How to fetch its related database value into texbox in datagridview coloumn and also I have to take 2 textboxes in datagridview coloumns and how to take multiplication of that datagriview coloumn value into another textbox in datagridview coloumn