Not sure you have it right. Did you mean 3y = x? If so, then you want to solve for y since a line is in the format y = mx + b where m = slope and b = constant.
f(x) 3y = x
y = 1/3 x (divide 3 by both sides)
This is the equation for a line with slope = 1/3 and no constant in the equation. Points on the line would be:
(0,0), (3, 1), (-3, -1), You only need 2 points for the line, but these three will make it easier to show where the lines goes on each side of the y axis. Remember (x,y) coordinates mean x units to the left or right and y units up or down.