How do you calculate that?

In order to be able to add/subtract two fractures, the denominators must be the same, i.e. You’ve got to find the chief man here. In this case, the “smallest possible” is desired (you could also simply expand the first break with the denominator of the second and the second break with the denominator of the first one, but with, for example, 1/2 + 1/4 you don’t…).

To do this, you “factorize” the denominators and see what factors may already be in both denominators, and then expand with the factors that lack one break compared to the other. At 1/2+1/4 you have only the 2 and 4=2*2 at the front in the denominator, i.e. in the first denominator is a 2 too little, so you would expand there with 2 and come to 2/4+1/4 and can then summarize.

In your example, you can clip the 3 in front of the denominator and binom in the second denominator. Use formula. That is, you get 3(x+y) on the one hand and on the other (x+y)(x-y). That is, the “smallest” main user is 3(x+y)(x-y). And that means that in the first denominator the factor (x-y) is missing, i.e. You have to expand this break with it, and in the second, the factor 3 is missing, that is, this break with 3. Then you can combine both fractures that now have the same denominator.