To find the common denominator of fractions, the LCM process is used.
To illustrate;
Note that the denominators of both fractions are different, that is, 3 and 5.
To solve this kind of math problem, the first step is to take a common denominator for both fractions. In this example, a common multiple for 3 and 5 would now be 15 (that is, the least of ALL possible multiples of 3 and 5)
Hence, the next step would be to apply the LCM to the denominators of both fractions, as follows;
The word to fill into the gap in your question is COMMON.
You need to find the "common" denominator