first you have to find their lowest common denominator:
7, 14, 21
3, 6, 9, 12, 15, 18, 21
then you multiply the fractions by how many times you had to multiply to get the common denominator:
5/7 × 3 = 15/21
2/3 × 7 = 14/21
then add: 15/21 + 14/21 = 29/21
ANSWER: 1 8/21 (you cant simplify 8 by 21)