IM A FUCKING RETARD HELP

   Dim stringg, length, newstring, replace, initial, pos, first, second As String
        stringg = txtSentence.Text
        replace = txtSecond.Text
        initial = txtFirst.Text
        length = txtFirst.Text.Length
        pos = stringg.IndexOf(initial)
        first = stringg.Substring(0, pos)
        second = stringg.Substring(stringg, length + pos)
        newstring = first & replace & second
        txtResult.Text = newstring