Simple Doubt in Functions

Question

• How to find identity element of a function ?

                   Ex : f(x)= x+y-3

• How to find identity element of fog(x) ?

    please take an example and explain for fog(x)

Comments

What is g(x) ?

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Can u pls take one example and explain ?

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fog(x) = f(g(x)). You have given the f(x), what about the g(x)?

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Can u pls show  how to find identity element for the first part ?