What must be common about two algebraic fractions before they can be added or subtracted?

The two algebraic fractions can be added or subtracted only if they have the common denominators. So when adding or subtracting algebraic fractions the first thing that must be done is to take the LCM of the denominators and convert the denominators to same expressions. Only after this step the fractions can be added or subtracted. If this step is not performed, the operations cannot be performed correctly. This concept is similar to the addition and subtraction of simple fractions. 

21

step-by-step explanation:

21-4