Fractions that are added or subtracted to one another must have the same denominator (common denominator).

In problems where the denominators differ from each other, a common denominator must first be found, before adding or subracting the fractions.

Here is one way to find a common denominator:

Multiply the two denominators together. The numerator of each fraction must also be multiplied by the same number as the denominator was mulitiplied by, so that the value of the fraction stays the same.