First, you find the greatest common divisor or better known as GCF. Then add the numerators, with the denominators still the same. Then simplify the number into lowest terms.
Example: 5/9 + 6/18 = ?
1. Find the GCF of 9 and 18. So, 9 & 18's GCF = 9.
2. After that, since you know that the GCF of both fractions is 9, add the numerators, with the denominators still the same. So, 5/9 + 6/9 = 11/9
3. Reduce to lowest terms. NOTE: Your fraction is a improper fraction. Change into a mixed number first then simplify. So, 11/9 = 1 1/9.
Hope this helped. :D