how can I dismiss first vc that second has presented modally on the first vc

I have a view controller that named "firstVC" and another one that named "secondVC" . the secondVC will present modally on the first vc I want to dismiss firstVC and secondVC when user push the button on the secondVC I call the method here But it does execute But nothing will happen in the secondVC this will happen

firstVC.dismiss(animated: true, completion: nil)

if you suggest me to use delegate please tell me how can I use that completely ? I don't know what should I put in the firstVC or what should I put in the secondVC

one more thing

In some other cases I need to execute json in the firstVC and when json has complete the both view controllers (firstVC and secondVC) be dismiss how can I do that too ?

Here is a solution using delegate:

protocol DismissDelegate{
    func dismissVC()
}

class FirstViewController: UIViewController, DismissDelegate{

    func showSecondVC(){
        let secondVC = SecondViewController()
        secondVC.delegate = self
    }

    func dismissVC(){
        self.dismiss(animated: true, completion: nil)
    }
}


class SecondViewController: UIViewController{
    var delegate: DismissDelegate?

    @IBAction func buttonAction(sender: UIButton){
        self.dismiss(animated: true, completion: nil)
        delegate?.dismissVC()
    }
}