Question

can anyone give me the answer that
how Einstein derive the speed of light c.?

Answer 1

Einstein did not derive the speed of light. That was done by James Clerk Maxwell, who had worked very closely with Michael Faraday on the Electro-Magnetism concepts. Both men are my two favorite Physicists, I strongly recommend a personal study of them, and EM.

Answer 2

So Einstein only gave the Relativity theory

Answer 3

Einstein did a great number of things in 1905, his "miracle year"; including writing the paper that won him the Nobel Prize and dealt with his work on the "photo-electric effect" . He is most commonly known for Special and General Relativity though. Search, "Einstein's Miracle Year" to learn more.