Piecewise-Defined Functions

Let

$$f(x) = \begin{cases} 3x^2 + 2&\text{if } x\le 3, \\ ax - 1 &\text{if } x>3. \end{cases}$$

Find $a$ if the graph of $y=f(x)$ is continuous (which means the graph can be drawn without lifting your pencil from the paper).