How to call class method in plugin wordpress

I have this code

function woocommerce_robokassa(){

        if (!class_exists('WC_Payment_Gateway'))
                return; 
            if(class_exists('WC_ROBOKASSA'))
                return;     

         class WC_ROBOKASSA extends WC_Payment_Gateway{

          public function X {}

        }
    }
 function add_robokassa_gateway($methods){
  $methods[] = 'WC_ROBOKASSA';
   return $methods;
    }

  add_filter('woocommerce_payment_gateways', 'add_robokassa_gateway');

Now I have to call X function her, but when I do it in this way

if(isset($_POST['newSum']) && !empty($_POST['newSum'])) {

  $myClass = new WC_ROBOKASSA();
   $myClass->X();
}

I get errore class WC_ROBOKASSA is not defined, how can call X function