In a problem such as this:

Sketch the region enclosed by the curves and compute its area as an integral along thex- ory-axis.y+x= 5,y-x= 0,y+ 3x= 4

or this:

Sketch the region enclosed by the curves and compute its area as an integral along thex- ory-axis.x=y3 - 18y,y+ 6x= 0

How do I find the limits of integration?