how to get subtotal from order factory using coupon filter in magento 2

Question

 $coupon='NL01-5234';//$_SESSION["code"] ;

        $collection = $this->_orderCollectionFactory->create()
            ->addAttributeToSelect('subtotal')
            ->addFieldToFilter('coupon_code', $coupon)//Add condition if you wish
            ->load();
        foreach ($collection as $c){
            print_r($c);

         }

How i get SUBTOTAL VALUE ?