Begin your search using the Discovery box, as above. This time select two of the keywords you found in your background reading. For example, you could begin your search using the keywords gangs and firearm sales.
The example below uses gangs AND firearm sales. When you click enter or search you will see a list of articles. Use the pane on the left side of the screen to limit your search by:
In the screen capture below, the limits are noted in red. Notice also where "subjects" is circled. Each article has a list of words that have been tagged to the article. These are "official" terms that work something like hashtags in Twitter. These subject words are a great source of additional keywords to test in your searches. Since the official subject term used for firearms sales is firearms trafficking, the search term could be changed to firearms trafficking.