In this equation as my point of view 
polynomial  is that equation  where x is a variable and it has power 2
and in equation there is not a such variable
No , it is not a equation , its a linear functional relation. f(x) can be anything. We have to determine the functin of f(x). for instance, if I say f(x).f(y)=f(x)+f(y). Now, this, being a equation, holds true for all values of x and y. When we put x=y=0, we get f^2(0)=2f(0), So f(0)=0 or 2. Now, after some more steps, we can find the vale of functiion.
ok i said that it is from my point of view
Its OK, By the way, I got the ans to this problem