Combinatorics is a part of math focused on counting principles of finite quantities. It does not really have much to do with triangles, much less the Pythagorean theorem.
As basic as combinatorics is, I feel that just the basic knowledge of the recognition of what a number actually is, would be more basic of a principle.
Combinatorics play an important role in Discrete Mathematics, it is the branch of mathematics ,it concerns the studies related to countable discrete structures. For more info, you can refer the link...