Chapter P - Section P.1 - Algebraic Expressions, Mathematical Models, and Real Numbers - Exercise Set - Page 17: 22

The set containing 3: {3}

The intersection of two sets can be found by looking for common elements of both sets. (elements that are in both sets) The first set is {1,3,7} and the second set is {2,3,8}. The only element that occurs in both sets is 3 and thus the answer is the set containing 3: {3}