If you carefully examine R, I think you'll find that the x-axis doesn't even touch it. If you look here, you can see that R is the region enclosed by the triangle. So, using this information, can you write down the integral?
I think you need to be absolutely sure which side of a line is in the allowed region. To do that, pick a point that is obviously on one side of a line, and plug in the coordinates of that point into the inequality, and see if it holds. Does this help?