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Can I assume that f(x)=mx+c?
I know the answer is 3. But, if the answer is 3. then, f(1) = 3 and f(f(1)) = 1 x 3 = 3 or f(3) = 3. Now, f(f(f(3))) = f(f(3)) = 3 x 3 = 9. But, we know f(3) = 3, So, f(f(3)) = f(3) = 3. So, What is the real value f(f(f(3))) is it 3 or 9? This function is not consistent. There are two values for f(f(f(3))) by two different approaches. Thus, this function and the question is invalid.