I am having difficulty figuring out this question:
There is a potential problem in determining whether or not a point is inside a polygon if one or more sides of the polygon are parallel to the scan line. How do I handle the difficulty. How do I cause the axis to be centered?

Since you’re obviously doing this as some kind of assignment, I suggest you look up the result in “Computer Graphics: Principles and Practice” section 3.6. If you’re taking a course on computer graphics, it should be available at the library. Who knows, you might learn something…