Which phrase best describes how to handle a subtraction of fractions with different denominators?

Finding a common denominator before subtracting fractions is essential because it allows for the fractions to be expressed in the same terms, making the operation valid. When fractions have different denominators, they represent different "whole" parts, which can lead to incorrect results if directly subtracted.

To subtract fractions, the first step is to determine a common denominator, which is typically the least common multiple (LCM) of the two denominators. This process transforms both fractions so they have the same denominator, which ensures that you are subtracting like quantities. Once both fractions are converted, you can subtract the numerators and keep the common denominator the same, leading to an accurate result.

This method is critical in maintaining the integrity of the mathematical operation and ensuring that the correct relationship between the fractions is upheld.