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?

let $\epsilon>0$,let $\delta = min(\frac{\epsilon}{7},1)$.
let $x$ satisfy $|x-3|<\delta$, if $x \in Q $, then we have:
$|F(x) - F(3)| = |x^2-9| = |x-3||x+3| \leq 7|x-3| < 7\delta \leq \epsilon$
if $x$ not in $Q$ we have:
$|F(x) - F(3)| = |3x-9| = 3|x-3| < 3\delta \leq 3 \frac{\epsilon}{7} < \epsilon$
so, for all x that satisfy $|x-3|<\delta$ we have $|F(x) - F(3)|<\epsilon$,
from here we see that $F(x)$ is continuous on x=3.
