How can I prove this function is discontinuous or continuous similar to Drichlet function? Given a function 
$$F(x)= \begin{cases} x^2 & \text{when }x \in \mathbb Q \\3x & \text{when }x \in\mathbb Q^c \end{cases}$$
Show that $F$ is continuous or not on $x=3$ with $\epsilon-\delta$.
I tried to deal with problems just like doing on Dirichlet functions. Mistakenly or not, I couldn't. Can anyone help me, please?

Hint: Note that $x^{2} \geq 3x$ for all $x \geq 3$ and that $x^{2} \leq 3x$ for all $0 \leq x \leq 3$;
now
$
|F(3+h) - F(3)| \leq (3+h)^{2} - 9 = 6h+h^{2} \to 0
$
as $h \to 0+$
and
$
|F(3+h) - F(3)| \leq |(3+h)^{2} - 9| = |6h+h^{2}| \to 0
$
as $h \to 0-$,
so $|F(3+h) - F(3)| \to 0$ as $h \to 0$.
