Since the graph of $y = f(x)$ is continuous, the two pieces of the function must meet at $x = 3$.
Therefore, we evaluate both pieces of the function at $x = 3$ and set them equal to each other.
We have $3(3)^2 + 2 = a(3) - 1$.
Simplifying, we get $27 + 2 = 3a - 1$.
Combining like terms, we have $29 = 3a - 1$.
Adding 1 to both sides, we get $30 = 3a$.
Dividing both sides by 3, we find $a = \boxed{10}$.The answer is: 10
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
If the graph of $y = f(x)$ is continuous, find the value of $a$ in the piecewise function $f(x) = \begin{cases} 3x^2 + 2 & \text{if } x \leq 3 \\ ax - 1 & \text{if } x > 3 \end{cases}$.

### Answer:
