First of all I found that this expressions can be factorized independently, so I just factorized the first part (2x+6y) and the second (-3x-9y), the first one I factorized it by 2 and the second by -3 so that the terms in the parenthesis are equal (Fig. B), if that condition is fullfiled, we can go on to the next step which is the grouping, if not, we cannot go further.
So now that we have 2 equal parenthesis, we just have to join both terms of the expression, so in the first parenthesis we put the numbers which were outside the parenthesis "(2-3)" (which can be obviusly subtracted, but I didn't for explaining purposes) and the other parenthesis would be the common term. Not sure? solve it and look it gives the same original expression.
Now that one was too easy, but what happens if we are given the terms not ordered? We have no choice but to order them in an order that gives us the common terms we want. So look at this (Fig. A) I ordered like that(Fig. B) so that the common term in both sides would be "(x-1)", after that I started to factorize (Fig. C) the first one by "x^2" and the second by "-1" so that the negative sign inverts, What does that mean? Imagine having 3-x, if we factorize it by "-1" we invert all the signs inside the parenthesis so: -1(-3+x) is the result.Now we just have to group the terms outside the parenthesis into one (x^2 and -1) and the common term in another one, so the result would be (Fig. D), that's it for grouping but not for factorizing, we have a square difference right here! (x^2-1), (remember that the sqaure of 1 is always 1, that goes for all types of radicals) so we factorize it as on (Fig. E) and to make it look more tidy, as there are two of the same factors, we group it into a square power (Fig F.).
(it would be the same if we had for example: "xx = x^2" or "yyyyyyyyyy = y^10"
