First off 3/9 and 6/9 are already common denominators. In some fractions you would need to find the least common denominator like in a problem like 1/2 + 4/5. Also 3/9 plus 6/9 results in 9/9 and 9/9 equals 1. Most fractions adding it subtracting results in a pan answer as a fraction but in this case it’s a whole number.