As we do it with rational numbers,

the first thing to do when we want to add or subtract a rational number to another is to find their common denominator,

so that we could simply add or subtract their numerators.

For example:

This is the same with rational expressions: first we are going to find a common denominator.

Than multiply the numerators of each fractions by the same factor their denominators have been multiplied so that they will have a common denominator.

Example: