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Ahhh very nice...

E2=m2c4+p2c2

If you say p is the momentum we can take p=0 for a mass at rest, right?

This gives us:
E2=m2c4+02c2

Ergo:

E2=m2c4

Take the square root and we got:

E=mc2

Thank you for this really good and inspiring information.

Yep, you got it right! :)