To graph an inequality, try to manipulate it into the form y \le f(x)y≤f(x) (or y \ge f(x)y≥f(x)). Once this is done, the points which solve the inequality will be those under (or above) the graph of the function: in fact, y=f(x)y=f(x) are exactly the points of the graph. This means that y \le f(x)y≤f(x) represents all the point below the graph, and vice versa y \ge f(x)y≥f(x) represents all the point above the graph.
In this case, you only need to divide by 9 both sides to get
y \le 12/9 xy≤129x which is equivalent to y \le 4/3 xy≤43x. So, you only need to draw the y=4/3 xy=43x line, and take all the part of the plan below the line.
Drawing such a line is quite easy, since it crosses the origin (you can see that (0,0)(0,0) fits in the equation, as 0=4/3*00=43⋅0), and a second point, chosen as you want. For example, if x=3x=3 you get y=4y=4. So, the line passes through the two points (0,0(0,0 and (3,4)(3,4), and any line is completely determined by two of its point.
Take a look at the graph: graph{9y\le 12x [-10, 10, -5, 5]}
