is this elimination used only in 3*3 matrix?
another question how did Gauss get to this answer?
No, Gaussian elimination can be used with any matrix or any system of equations.
Gaussian elimination is basically applying the same steps to the matrix that you would to solve a system of equations- in fact, Gaussian elimination was first applied to systems of equations rather than matrices.
halls of ivy would you happen to know the answer to my second question?
Yes. In fact, I gave it. Did you notice that my answer had two paragraps? First paragraphs answers first question. Second paragraph answers second question.
Separate names with a comma.