I am looking at a derivation of the parallel axis theorem in a book. The photo is below
I'm not sure why but I just cannot figure out how they got from line 2 to line 3. I have thought through all of the properties I know (I have expanded and used the vector triple product, the cyclic permutations of scalar triple product etc) but I cannot seem to figure out that step.
Would very much appreciate if someone could enlighten me as to what property/rule was used here.