Be careful with your parentheses there. is very different from Which one did you mean?
Generally, I'd simply use the calculus definition of continuity here: make
happen.
Based on your subsequent post, this is incorrect. You mean (ax-4)/(x-2).
Using universally accepted order of operations,
ax-4/x-2 means
and
ax-4/(x-2) means
and
(ax-4)/(x-2) means
It wouldn't be a bad idea to learn how to use LaTeX either. This code
[tex]f(x)=\begin{cases}\dfrac{ax-4}{x-2}&,\ x\ne2\\\\b&,\ x=2\end{cases}[/tex]
produces
As to your question.. like Ackbeet said, you need . In order to have this you will need to follow Jhevon's advice.