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.
-149 would be your answer
2/3n (put that as a fraction) + 4 = 7