What you have are sets A and B and then the Union of those 2 sets. The Union of sets is like the "marriage" of them. The 2 sets come together and bring along with them ALL of their junk and they have to share it.
The upside down U indicates the intersection of the sets. In other words, what they both have in common. They both share a 5 and a 7. Therefore, A∩B = {5, 7}
