It's important to understand the difference:
* Discover: Finding something that already exists.
* Derive: Using existing knowledge and logic to arrive at a new conclusion.
Einstein's contribution was to show that mass and energy are equivalent and related by the famous equation, but the concept of mass-energy equivalence had been hinted at before by other physicists.
Therefore, while Einstein is credited with the derivation of E=mc², it's not accurate to say he "discovered" it.
