How to auto calculate from database?

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ASP.NET

Hello.

I am trying to auto calculate from the database. What I have is TextBox1, TextBox2 and TextBox3. I am populating TextBox1 and TextBox2 from the database and I have the TextBox_Text change on Textbox2. How can I get the two TextBoxes to calculate the numbers from the database? This is the code I have in place. I also have AutoPostBack on TextBox2.

using System;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Text;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Configuration;
using System.Drawing.Printing;
using System.IO;
using System.Web.SessionState;
using CrystalDecisions.CrystalReports.Engine;
using CrystalDecisions.Shared;

namespace SACSCOCLogin1._1
{
    public partial class TestCal : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["HotConnectionString"].ConnectionString);
            con.Open();

            SqlCommand scmd = new SqlCommand("Select Cars, Trucks from TestCal", con);
            SqlDataReader dr = scmd.ExecuteReader();

            if (dr.Read())
            {
                TextBox1.Text = dr["Cars"].ToString();
                TextBox2.Text = dr["Trucks"].ToString();
            }
            dr.Close();
            con.Close();
        }

        protected void TextBox1_TextChanged(object sender, EventArgs e)
        {
            int a = Convert.ToInt32(TextBox1.Text);
            int b = Convert.ToInt32(TextBox2.Text);
            TextBox3.Text = Convert.ToString(a + b);
        }
    }
}