Looking at the attachment, we can divide the area that they enclose into 2 parts:

(1) The area enclosed by the blue line (y = 2 - x) and the yellow curve (the parabola) from their intersection point at the very left to the intersection point of the blue and purple line.

(2) The area enclosed by the purple line and the yellow curve from the intersection of the blue and purple line to the intersection point of the purple line and the yellow curve at the far right.

Try to apply the 'upper - lower function' to both these cases and see what you can come up with!