What is the basic principle that can be used to simplify a polynomial?
Isn't that factoring?
Grouping and combining of like terms is the first step.
For example,
3x²+2x-5+3x²+4-3x
= 3x²+3x² +2x-3x -5+4
= 6x² -x -1
After that, factoring can play a role.
6x² -x -1
=(3x+1)(2x-1)