Feed RSS doesn't work

Hi! I tried to add following code in my theme function.php file. Update, but doesn’t work my blog. Delete code, and after, feed don’t work. This is code that try to add.

function rssthumb($content) {
	global $post;
	if(has_post_thumbnail($post->ID)) { ?>
		<img align="left" width="75" src="<? echo wp_get_attachment_thumb_url( get_post_thumbnail_id( $post->ID ) ) ?>" hspace="10" />
		<? }
	return $content;
}
add_filter('the_excerpt_rss', 'rssthumb');
add_filter('the_content_feed', 'rssthumb');

This code is for add images in feed.
Now, when check the url, see that. ropetili.eu/feed/
W3.org validator says W3.org validator