Prove that $A \oplus B \simeq B \oplus A$ using natural transformation

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I have to prove that $A \oplus B \simeq B \oplus A$ where $A,B$ are $R-$ Modules using natural transformation. I defined $\varphi: f \to g$, where $f,g$ are functors of two variables. I defined $f(A,B)= A \oplus B$ and $g(A,B)= B \oplus A$, but I don't know how to prove that $\varphi$ is an equivalence