The square of a formal laurent series

[R.P.F.]

Let $$F$$ be a field. Let $$c \in F$$.

I am trying to show that if
$$c = f^2$$ where $$f\in F[[x]]$$, then $$f\in F$$.

So I am able to get rid of the terms with negative exponent. So now I'm left with a formal power series. Anyone knows how to do this? Thanks!

 

[R.P.F.]

Figured it out. So sorry, guys. :(