private void Divisionbutton_Click(object sender, EventArgs e)
 {
     try
     {
         operation   = ":";
         firstNumber = float.Parse(ScreentextBox.Text);
         ScreentextBox.Clear();
     }
     catch
     {
         ScreentextBox.Text = "SYNTAX ERROR";
     }
 }
 private void ACbutton_Click(object sender, EventArgs e)
 {
     ScreentextBox.Clear();
 }