Albert Einstein's most famous equation expresses the relationship between the energy (E) of a system, the mass involved (m) and the speed of light (c).
Your question asks how the "c" was determined; in other words, how was the speed of light first calculated. Although Galileo had guessed that light transmission was not instantaneous, as was commonly thought, he could not prove it. In 1676 a Danish astronomer named Olaus Roemer saw that there were differences in the times between what was predicted and what was observed for eclipses of Jupiter's moons. He realized that because of differences in the distances between the Earth and Jupiter during different parts of their orbits that the time light spent traveling from Jupiter to Earth would vary. Although astronomers at the time were unable to accurately measure the distance between the planets, Roemer used his best estimates and calculated the speed of light to be 214,000 kilometers per second. The link I have attached details further experiments measuring the speed of light. The current accepted value is 299,792.458 km/s.
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