Parallel lines share the same slope value. To solve this you first need to convert the equation into slope intercept form. (y=mx + b)
Do this by subtracting 3x from each side leaving
y = -3x + 9
In the y=mx + b equation m is your slope.
Any line with a slope of -3 will be paralell to this line. The b value which is the y intercept will change and move the line up or down on the y axis. An example of a parallel line would be
y = -3x +2
This line would have the same slope but would shift down the y axis 7 spots.