Slam

I cant seem to make the gallery shown in the attachment ive marked up be the same width as the image blocks below it. The 4 blocks below the gallery appear to be full container width at any scale. However, whenever you scale the browser up, the gallery block (in red) looks like it has about a 20px margin on both sides and I cannot figure out which style is controlling that. I want the gallery to be the same width as the 4 blocks below it any browser window size. I appreciate any help. Thanks