How do you find the constant of variation when y=-(2/3) and x=3

You can't. If you think about the straight line on a graph, those numbers describe a single point that the line goes through, and they don't tell you anything about the slope of the line, or where it crosses the x-axis or the y-axis.  So I don't think you can tell the constant of variation from one point.

[tex]y=-\frac{2}{3};\ x=3\\\\kx=y\\\\3k=-\frac{2}{3}\ \ \ \ /:3\\\\k=-\frac{2}{9}[/tex]