Video #1 - How do you find the common denominator and the least common denominator?
http://www.virtualnerd.com/middle-math/adding-subtracting-fractions/unlike-denominators/common-and-least-common-denominator
Video #2 - How do you add fractions with unlike denominators?
http://www.virtualnerd.com/middle-math/adding-subtracting-fractions/unlike-denominators/practice-add-fractions-unlike-denominators