we need to show it is discontinuous at x≠0 can any one just explain to me to me answer

Q)
   $$f(x)=\begin{cases}
x &\text{if }x\in \mathbb{Q} \\
0 &\text{if }x\in \mathbb{R}\setminus\mathbb{Q}
\end{cases}$$
  we need to show it is discontinuous at x≠0 
Answer
  Let us test the discontinuity at x=c, where c in Q and $c≠0$.
  $f(c+)=\lim f(x)=\lim f(c+h)-f(c)\backslash h$ = $$f(x)=\begin{cases}
\lim ((c+h)-(c))\backslash h &\text{if }c+h\in \mathbb{Q} \\
\lim ((0)-(c))\backslash h,doesnot exit  &\text{if }c+h\in \mathbb{R}\setminus\mathbb{Q}
\end{cases}$$
  where is 
  $x\rightarrow c+$

$h\rightarrow 0$
my question
this is the definition right of right side limit , please tell me this definition right or wrong?
what is h ?

If function $f(x) $ is not continuous
at $x = a$ then we can construct a sequence $(x_n)$ which converges to $a$ but for which
$f(x_n)$ does not converge to f(a).
Take two sequences $(x_n)$ and $(y_n)$ of rationals and irrationals, respectively  and think about their images.
