Find the value of the constant $b$ that makes the following function continuous on $(-\infty,\infty)$. $$f(x) = \begin{cases}\displaystyle{3x-4}&\text{if}\ x \le 9\cr \displaystyle{-3x+b}&\text{if}\ x > 9\end{cases}$$

$b=$

Now draw a graph of $f$.