How much more pounds in total were transported to New York than to Chicago?
"New York" and "Chicago" are both city_name; more pounds in total refers to Subtract (Sum(weight where city_name = 'New York'), Sum(weight where city_name = 'Chicago'))
SELECT SUM(CASE WHEN T2.city_name = 'New York' THEN T1.weight ELSE 0 END) - SUM(CASE WHEN T2.city_name = 'Chicago' THEN T1.weight ELSE 0 END) FROM shipment AS T1 INNER JOIN city AS T2 ON T1.city_id = T2.city_id