How to get a drop down list box to populate textboxes in asp.net

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Drawing;
using System.Text;
using System.Data;
using System.Collections.Generic;
using System.Linq;
using System.Data.SqlClient;
using System.Configuration;


public partial class FinanccialIndicatorsForm : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["PassConnectionString"].ConnectionString);

        SqlCommand scmd = new SqlCommand("Select FINYR from TableFIN where TOTASSETS, TOTLIABILITY and NONEXPPERMRESASSETS = " + DropDownList1.SelectedValue.ToString() + "@TOTASSETS, @TOTLIABILITY and @NONEXPPERMRESASSETS", con);

//this is what it needs to look like. just replace your values you want where the labels are
      SqlCommand scmd = new SqlCommand("Select FINYR from TableFIN where TOTASSETS = '" + [add value here] + "' and TOTLIABILITY = '" + [add value here] + "' and NONEXPPERMRESASSETS = '" + [add value here] + "'", con);

        

        try
        {

            con.Open();

            SqlDataReader sdr = scmd.ExecuteReader();

            TextBoxLYTA.Text = sdr["TOTASSETS"].ToString();

            TextBoxLYTL.Text = sdr["TOTLIABILITY"].ToString();

            TextBoxLYNPRNA.Text = sdr["NONEXPPERMRESASSETS"].ToString();
        }

        catch (Exception ex)
        {
        }

        finally
        {
            con.Close();
        }
    }
    protected void ButtonSubmit_Click(object sender, EventArgs e)
    {
        SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["PassConnectionString"].ConnectionString);
        con.Open();


        string insCmd = "Insert into TableFIN (TOTASSETS, TOTLIABILITY, NoNEXPPERMRESASSETS, UNRNETASSETS, TOTALREV, TUITFEES, CURRDEBT, LOMGTERMDEBT) values (@TOTASSETS, @TOTLIABILITY, @NoNEXPPERMRESASSETS, @UNRNETASSETS, @TOTALREV, @TUITFEES, @CURRDEBT, @LOMGTERMDEBT)";
        SqlCommand insertUser = new SqlCommand(insCmd, con);
        insertUser.Parameters.AddWithValue("@TOTASSETS", TextBoxTA.Text);
        insertUser.Parameters.AddWithValue("@TOTLIABILITY", TextBoxTL.Text);
        insertUser.Parameters.AddWithValue("@NoNEXPPERMRESASSETS", TextBoxNPRNA.Text);
        insertUser.Parameters.AddWithValue("@UNRNETASSETS", TextBoxTUNA.Text);
        insertUser.Parameters.AddWithValue("@TOTALREV", TextBoxTR.Text);
        insertUser.Parameters.AddWithValue("@TUITFEES", TextBoxTFN.Text);
        insertUser.Parameters.AddWithValue("@CURRDEBT", TextBoxCD.Text);
        insertUser.Parameters.AddWithValue("@LOMGTERMDEBT", TextBoxLTD.Text);

        try
        {
            insertUser.ExecuteNonQuery();
            con.Close();
            Response.Redirect(".aspx");
        }
        catch (Exception er)
        {
            Response.Write("You Have Successfully Submitted the Information!!!");
        }
        finally
        {

        }
    }
}