The field $C$ is an algebraic closure for $F$ Let $E/F$ be an algebraic extension and $C$ be an algebraic closure of $E$.
I want to show that the field $C$ is an algebraic closure also for $F$. 
I have done the following:
$C$ is an algebraic closure of $E$, i.e., each polynomial of $E[x]$ has all the roots in $C$. Let $f\in F[x]$ be a non-zero polynomial. 
Since $E$ is an extension of $F$, we have that $f\in E[x]$. 
So, each polymnomial of $F[x]$ has all the roots in $C$. 
To conclude that $C$ is an algebraic closure also for $F$, it is left to show that $C/F$ is algebraic, or not? 
So, we have to show that each element of $C$ is algebraic over $F$, i.e., for each element $c\in C$ there is a non-zero polynomial $g(x)\in F[x]$, such that $g(c)=0$. 
To show that we have to use the fact that $C/E$ is algebraic, or not? But how exactly? 

Suppose $F \subseteq E \subseteq L$ are fields.  If $L$ is algebraic over $E$, and $E$ is algebraic over $F$, then $L$ is algebraic over $F$.  This result should give you what you want.
Here is a sketch of the proof.  In general, if $A \subseteq B$ are fields, and $b \in B$ is algebraic over $A$, then $A(b)$ (by definition, the intersection of all subfields of $B$ which contain $A$ and the element $b$) is a finite extension of $A$, i.e. it is finite dimensional as a vector space over $A$.  It follows that if $b_1, ... , b_n \in B$ are algebraic over $A$, then $A(b_1, ... , b_n)$ is a finite extension of $A$.  This follows because $A(b_1, ... , b_i) = A(b_1, ... , b_{i-1})(b_i)$ is a finite extension of $A(b_1, ... , b_{i-1})$.  
If $x \in L$, then $x$ is the root of a polynomial $f(X) = a_0 + a_1X + \cdots + a_nX^n \in E[X]$.  Now all the $a_i$ are algebraic over $F$, so $F(a_0, ... , a_n)$ is a finite extension of $F$.  Clearly, $x$ is algebraic over $F(a_0, ... , a_n)$, so $F(a_0, ... , a_n)(x) = F(a_0,..., a_n,x)$ is a finite extension of $F(a_0, ... , a_n)$, hence of $F$.  
In particular, $x$ is a member of a field which is a finite extension of $F$.  Now, you just need to show that finite extensions are algebraic.
