To solve this problem, you have to find the common denominator. Since 3 and 7 share no common factors, you can just multiply them together. The common denominator would be 21. 1/3 = 7/21 and 3/7 = 9/21.
7/21 + 9/21 = 16/21. Since 16/21 is already in simplest form, that is the final answer.
Hope this helped! :)
