How to calculate the values in each row through an equation in C#?

i need help. I tried to solve an equation through the values in each row in the table, so i tried but i didn't get the solution.

However, I supposed to calculate the avg, when the item is y, ( balance = previous balance - y's qty ) , ( price = X's Average, Avg= X's Avg).

While the item is X, ( balance = previous balance + qty) , ( Avg = (previous balance * previous Avg) + (price * qty) / balance.

The table that shows what i supposed to:http://imgur.com/Ch7jDyP

The Code:

Puvlic void Total()
{
try
            {
                string x = string.Empty;

                string ccc = WindowsFormsApplication5.Properties.Settings.Default.CCCConnectionString1;

                x = "SELECT Distinct * FROM Test$";
//Table Name = Test$
                SqlConnection c = new SqlConnection(ccc);
                DataSet ds = new DataSet();
                DataTable dt = new DataTable();
                using (SqlDataAdapter da = new SqlDataAdapter(x, c))
                {

                    c.Open();

                    da.SelectCommand.ExecuteNonQuery();

                    da.Fill(ds);
                    da.Fill(dt);
                    foreach (DataRow row in ds.Rows)
                        {
                            //row[6]=qty
                            //row[7]=price
                            //row[8]=balance
                            //row[9]=avg    

                      

    

                        int ybalance = Convert.ToInt32(row[8]) - Convert.ToInt32(row[6]);
                                
                        int xbalance = Convert.ToInt32(row[8]) + Convert.ToInt32(row[6]);
                        double TotalCost = Convert.ToInt32(row[6]) * Convert.ToDouble(row[7]);
                        double avgcost = Convert.ToDouble(row[9]);
                        double avg = ((ybalance * avgcost) + TotalCost) / xbalance;

                           
                        
                        if  (Convert.ToInt32(row[7]) == 0)
                            
                        {
                            row[8] = ybalance;

                            row[7] = avgcost;

                            row[9] = avgcost;
                        }

                        else
                        {
                            row[8] = xbalance;

                            row[9] = avg;


                        }



                            }
                        }
                                        
                    c.Close();
                    label1.Text = "Done";

            }

            catch(Exception f)
            {
                label1.Text = "Failed" + "Error: " + f.Message;

            }
        }

private void button1_Click_1(object sender, EventArgs e)
        {
            Total();

           
        }

Firstly, I tried to let them null but it returned DBNull Error. Then i convert the null values to Zero, it executed but the values remained 0s and didn't follow the equations. Also i put the break-point at da.fill(ds) it retrieved values but i couldn't find them in table data.  Any Suggestions or help guys? 
Thanks :) ,