you're calling mysql_query two times, you should not do that. Instead you should directly assign the return value of the first mysql_query call to $result like so:
$result_=_mysql_query("SELECT Item_id FROM products ORDER BY Item_id DESC LIMIT 1");
Though you will have to make another call to fetch the actual data from the result. with one of the mysql_fetch_* calls, take a look at the php manual here
btw, do try to use a more descriptive subject line next time, Thank you.