Please help me in calculating grossrent?

i attach file please see.


rent textbox 100   in that i have calculate button when i click the calculate button 10 will display in tax text box(10)

tax  textbox 10
 after that i want to add 100 and 10 
grossrent textbox 110
please give the code in csharp i dont know how to do .please send the code.

And copy the below code as follows

  if (txt_rent.Text != "")
                txt_tax.Text = (Convert.ToDouble(txt_rent.Text) * 0.1236).ToString();