x^3 plus some other stuff can be broken into (x+1)(x+2)(x+3) at the most because there are only 3 x's kind of, because x^3 can only be broken down into 3 x's, if you get what I am saying so therefore x^n can only be broken down into n number of x's
I hope you find this helpful, I'm not too good at explaining stuff, but I like to try, I wasn't sure if you needed an official-y proof, cuz i'm way too lazy to do that for ya