That's not the Klein-Gorden equation. It's the relativistic relation between energy, mass and momentum. Also called the relativistic dispersion relation.
In general, you should accept a lot more type-ins. I'm a theoretical physicist and knew every equation, but only got 11/29 when my time ran out, because I had so much trouble getting the exact version you wanted to hear.
In general, you should accept a lot more type-ins. I'm a theoretical physicist and knew every equation, but only got 11/29 when my time ran out, because I had so much trouble getting the exact version you wanted to hear.