use  MagentoStoreModelStoreManager $storeManager

    $this->storeManager = $storeManager;
    $config_path =
    catalog/placeholder/thumbnail_placeholder OR
    catalog/placeholder/swatch_image_placeholder OR
    catalog/placeholder/small_image_placeholder OR
    catalog/placeholder/image_placeholder OR

     public function getConfig($config_path)
        {
            return $this->storeManager->getStore()->getConfig($config_path);
        }
    $mediaUrl = $this ->storeManager->getStore()->getBaseUrl(MagentoFrameworkUrlInterface::URL_TYPE_MEDIA );


   <img src = $mediaUrl.'catalog/product/placeholder'.$this->getConfig($path) />