First of all, notice that the given line passes through the origin, (0,0). Because of this we know immediately that the y-intercept of this line is (0,0); alternatively, "b" in "y=mx+b" is zero: Thus, the equation of the line has the form y=mx.
Next, determine the slope, m, of the given line. Slope m = rise / run. Here, if we start at (0,0) and move 3 spaces to the right along the x-axis, and from there 9 spaces down, we get m = rise / run = -9 / 3), or m = -3.
Thus, the equation of the given line is y = -3x.