The fractions 2/3 and 3/4 can be turned into fractions with a common denominator by finding the Least Cmmon Denominator, or the LCD. This means that you are finding the number where 3 and 4 first intersect. The easiest and most direct way to find a common denominator is by multiplying them by each other.
The answer for this will be 8/12 and 9/12.