Why would a different sort of boundary pose any different process for plotting? Just find teh border and decide which side.
x2 <= (1-x1)^3
Okay, have you ever seen the function y = x^3? It should look a lot like that, only shifted one (1) to the right and reflected across the x-axis (flipped vertically). Can you see why there are TWO variations from y = x^3?
Your last task is to decide which side to shade. You can either wring your hands and wonder or you can just TRY something. (0,0) is a nice place to start unless it is on the boundary. Is this true: 0 <= (1-0)^3. If so, shade that side.