Question

prove that velocity of light is 3×10^8 ?

Answer 1

It can be actually calculated from Maxwell theory (electrodynamics) via

$c=\frac 1{\sqrt{\varepsilon 0μ0}},$

where the two constants in the denominator are the dielectical and permeability constant of the vacuum. That tells it has to be 299,792,458 m/s, and many experiments have shown it to be that way since.