I am actually differentiating functions, but I get stuck on the part involving simplifying. Below is what the book says, but I cannot follow how they got from the first to the second part...

= 28(3x+1)^3 (7x-4)^3 + 9(7x-4)^4 (3x+1)^2

= (3x+1)^2 (7x-4)^3 [28(3x+1) +9(7x-4)]

Any help would be much appreciated!
Thanks.