get the new word from the second string

Question

I am trying to get the new words from the second string when match the same word(second string) from another string(first string)

 

Current output is:

 

Insert Word Is: matching hims by website

 

I want to:

 

Insert Word Is: by website

 

.cs

protected void Button1_Click(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        Label3.Text = "";
    }

    var result1 = stextbox.Text.Split(new char[] { ' ' }).Except(ftextbox.Text.Split(new char[] { ' ' })).ToArray();
    int count = 0;
    count = result1.Length;

    //string lastWord = result1[result1.Length - 1];

    for (int i = 0; i < count; i++)
    {
        Label3.Text += result1[i].ToString() + "   ";
    }

    Label3.Text = "Insert Word Is: " + Label3.Text;

    //below code is get the last word
   //var result1 = stextbox.Text.Split(new char[] { ' ' }).Except(ftextbox.Text.Split(new char[] { ' ' })).ToArray();
    //int count = 0;
    //count = result1.Length;

    //string phrase = ftextbox.Text;
    //string subPhrase = stextbox.Text;
    //subPhrase = phrase;
    //string temp;
    //while (true)
    //{
    //    temp = subPhrase.Substring(1 + subPhrase.IndexOf(" "));
    //    if (temp.Equals(subPhrase))
    //    {
    //        break;


    //    }
    //    else
    //    {
    //        subPhrase = temp;
    //    }
    //}

    //Label3.Text = "Insert Word Is: " + temp;

.aspx

<asp:TextBox ID="ftextbox" runat="server" TextMode="MultiLine" Width="500px" Height="200px"></asp:TextBox>
<asp:TextBox ID="stextbox" runat="server" TextMode="MultiLine" Width="500px" Height="200px"></asp:TextBox>
<br /><br />
<asp:Button ID="Button1" runat="server" Text="Compare" OnClick="Button1_Click" />
<br />

need help