$F$ is algebraically closed $\iff$ $\nexists$ $K$ s.t. $F \leq K$, $K \neq F$ and $[K:F] < \infty$ 
$F$ is algebraically closed $\iff$ $\nexists$ $K$ s.t. $F \leq K$, $K \neq F$ and $[K:F] < \infty$.

Proof:
($\implies$) assume by way of contradiction that $F$ is an algebraically closed field and there does exist such a field $K$, say $[K:F] = n$.
Let $a_1,....a_n$ be a basis for $K$ over $F$.
Okay, now I'm lost, haha
$(\impliedby)$ 
I need help with this direction also.
Thanks all! You're the best!

The correct statement is : 

$F$ is algebraically closed if and only if there does not exist a finite field extension $K$ of $F$, i.e. there does not exist $K$ such that $F\leq K$ and $[K:F]<\infty$. 

Proof : $(\Rightarrow)$ Say $K$ is a finite field extension of $F$ of degree $n$. Let $\alpha\in K-F$. Then $\{1,\alpha,\alpha^2,\dots,\alpha^n\}$ is linearly dependent over $F$. Then there exists $\lambda_i\in F$ such that $\sum_i\lambda_i\alpha^i=0$, i.e. $\alpha$ satisfies a polynomial with coefficients in $F$, which contradicts the fact that $F$ is algebraically closed.
$(\Leftarrow)$ Say $F$ is not algebraically closed. Then there exists an irreducible polynomial $f\in F[x]$ such that $f$ has no root in $F$. Then $K=F[x]/(f)$ is a finite field extension of $F$, which is a contradiction. Hence $F$ is algebraically closed.
