It shows a sample formula which allows you to detect whether a line, defined by two points, intersects a rectangle. Please give it a try with your coordinates and see whether it would fit your requirement.
Note that MapTileSystemHelper offers the LatLongToPixelXY method which allows you to convert any PointG to PointL which represents a point in Cartesian coordinates represented with Int64 values. Thus, you can use the formula from the above referred StackOverflow thread.
I hope this information helps. If you need any further assistance please don't hesitate to contact me.
Dess | Tech Support Engineer, Sr.
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