Answer:
Isaac Newton's major discoveries in physics, including his laws of motion and his theory of universal gravitation, were made in the late 17th century, while Albert Einstein's major discoveries in physics, including his theory of relativity and his equation E=mc^2, were made in the early 20th century. Therefore, approximately 300 years elapsed between Newton's and Einstein's discoveries.
Newton was born in 1643 and made his major discoveries in the 1680s and 1690s. Einstein was born in 1879 and made his major discoveries in the 1905-1915 time period.