Swift - 如何从UITextField中乘以值

@IBOutlet weak var labelLabel: UILabel
@IBOutlet weak var textFieldField: UITextField
@IBAction func buttonButton(sender: AnyObject) {

   var conversionRate = 7

        self.labelLabel.text = self.textFieldField.text // Here I'm trying to multiply textFieldField by 7 (conversionRate)

        textFieldField.resignFirstResponder()
    }
@IBOutlet weak var labelLabel: UILabel
@IBOutle