To add this, find the LCD of these two fractions. In this case, the LCD of 7 and 2 is 14. Multiply 3/7 by 2/2, and 1/2 by 7/7 to get 6/14+7/14. When you add that together, you get 13/14. Check to see if you can simplify. For this number, you cannot, because 13 is a prime number.
Hope that answered your question.