Solving linear inequalities in two variable is very easy when you can write is as y\ge f(x)y≥f(x) (or y \le f(x)y≤f(x)).
In these cases, in fact, the graph of f(x)f(x) represents the points where y=f(x)y=f(x) holds. To solve y\ge f(x)y≥f(x) you'll need to consider all the points "above" the graph, and vice versa for y \le f(x)y≤f(x).
In your case, f(x)=3-xf(x)=3−x, which is a line. A line can be graphed once two of its points are known. You can choose two easy points by setting x=0x=0 (obtaining y=3y=3), and x=3x=3 (obtaining y=0y=0).
So, the points (0,3)(0,3) and (3,0)(3,0) belong to the line. Connect them to find the graph of the line, and consider all the points above the line to solve the inequality.
Here's the graph: graph{y \ge 3-x [-10, 10, -5, 5]}
