To determine the equation of a line parallel to $y = 3x + 9$ and passing through the point $(2, 1)$, we need to note the following:

1. Slope: A line parallel to another will have the same slope. Here, the slope of the line $y = 3x + 9$ is $3$.

2. Equation form: We use the point-slope form $y - y_1 = m(x - x_1)$, where $(x_1, y_1)$ is the point the line passes through and $m$ is the slope.

Using $(2, 1)$ as our point and $3$ as the slope:

$y - 1 = 3(x - 2)$

Expanding this:

$y - 1 = 3x - 6$

$y = 3x - 5$

So, the correct answer is Option C: $y = 3x - 5$.

Would you like a further breakdown of the steps?

Further Questions:

1. What is the significance of using the same slope for parallel lines?
2. How do you determine the slope of a line from its equation?
3. What are other forms of linear equations, and when are they used?
4. How can you verify if two lines are parallel or perpendicular?
5. What changes if we wanted a line perpendicular to $y = 3x + 9$ through the same point?

Tip: For lines to be parallel, their slopes must be identical.