Cutting Polygon

If I’m understanding you correctly the easiest way to do this would likely be something like this:
1. Start with a base 4 sided rectangular shape
2. Create another rectangle larger than the base shape. Rotate this shape by 5.5 degrees.
3. Overlap shapes such that the larger shape would remove the portion of the base shape you don’t want.
4. Select base shape
5. Add larger shape to selection
6. Tools->Boolean Difference
Something like this: