Question

why e=mc^2 but not e=mc . i mean how Einstein get there???

Answer 1

e=mc² was first derived from
m=e/c²

Answer 2

The real mass energy equivalence equation that einstein derived was

E^2 = (mc^2)^2 + (pc)^2

where p is momentum. But when the particle is at rest p is zero then we get the famous equation

E=mc^2

Also this equation is widely misunderstood. In the research paper which einstein published he wrote it in the form

m=E/c^2

Secondly this doesnot mean that Mass is commpressed energy or it is traped energy. This equation has a really deep meaning which shatters the concepts of mass and energy as we common people percieve it.