It is possible to add and subtract fractions containing different variables. For fractions with different denominators that are going to be added or subtracted together, you calculate the common denominator in the same way as you do with fractions that contain only numbers.
Write as a single fraction
Variables just represent numbers. Since is one denominator and is the other one, then is the common denominator between the two fractions. You find this by expanding the first fraction by , and the second fraction by :