Question

Find A ∩ B if A = {4, 7, 10, 13, 17} and B = {3, 5, 7, 9}.

Answer 1

The "intersection" of two sets is the set of all members of the 2 sets that are in common.  3 and 4 are not in common here; 7 is; all the rest are not in common.  Thus

A ∩ B = {7}