(Note: you could have made the problem lots easier by simplifying the fractions at the beginning into 1/3 and 1/3 and then just adding those, but that would defeat the purpose of learning how to add unlike denominators.)
Now, you have two fractions with the same denominator, so you can add them as normal: The least common multiple between 9 and 6 is 18 (you can learn how to find LCMs by using the search box in the top of any Khan Academy screen, if you don't know already). With all that out of the way, let's see that example:
The easiest way to convert two fractions to the same denominator is to make each denominator the least common multiple of the two previous denominators. To convert a fraction into a different denominator, you have to multiply the numerator and denominator by the same number (in order to keep the actual value the same). You would need to convert the fractions so that they have the same denominator, and then add them by adding the values in the numerators and keeping the denominator the same.
