This is the video about how to add a rational expression. So to add a rational expression first of all find the common denominator. To find the common denominator, just find the least common multiple (LCM) of the denominators. There are at least a couple ways of doing this. To convert each fraction to the common denominator, you multiply each denominator by what it needs in order to turn it into 6x. For instance, in the x+4/5x, the denominator needs to be multiplied by 6x, since 6xX5x = 30x. To keep things fair, you multiply the top by 6x as well. This is because 6x/6x = 1, and multiplying things by 1 doesn't actually change them. Converting the other fractions, you get the answer. That's it.