Is 72-(63-8) and (72-63)-8 communitive

What 2 forces allow things to fall ‍♀️‍♀️

You are assembling pieces of an iron gate to complete a fence. the finished gate will look like the one below. in order to assemble the gate it is important to under how the pieces are related. part 1: how are pies 1 and 2 in the archway related? part 2: how are the rectangular sections 5 and 6 in the center of the gate related to each other?

How is this equation completed? i cannot find any examples in the book.