To answer in the same style the question was asked at, my equation would be that "mathematics = number theory + geometry". I think it all stems from these two; the fact that this was historically so supports my view.

In an insightful comment to the original question, Mariano Suárez-Alvarez wrote: "I think that there are three basic forms of mathematics: number theory, combinatorics and topology". Our answers would be almost the same if we could classify combinatorics as part of number theory. I am certainly not competent enough to give an opinion as to whether that would be a fair classification (and I have a feeling that such a classification might hurt the feelings of some combinatorialists).