Given the function f : R^3 \rightarrow R^3, f(x,y,z) = (x+y, y-z, 2x+y+z) how can I prove that f \epsilon End_R(R^3) ?

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That's the problem, that I couldn't do anything. I don't know how to express the formula for endomorphism. I only know that the domain should be equal with the codomain and in this case is true, but that's all? I only need the hints for solving it, not the entire solution.