Answer:
neither
Here's how you know:
first, i'll simplify it so both are in slope-intercept form (y = mx + b).
x-3y=6
-3y = 6 - x
y = (1/3)x - 2
y=3x+4 is already in slope-intercept form, yay :)
these lines are neither parallel nor perpendicular.
The equations of parallel lines have THE SAME SLOPES (y = mx + b --> m would be exactly the same).
In contrast, perpendicular lines have NEGATIVE RECIPROCAL SLOPES (y = mx + b --> m would be -1/m).
--> since the slopes of these match neither of these definitions, we know it must be neither.