Can't figure out how to cut compound angle

An alternative method is to insert a Plane and use an Extruded Cut

  1. Step 1:

    Create a Plane. (Features, Reference Geometry, Plane) select 3 points on the sketch,

  2. Step 2:

    Start a new sketch and draw a large rectangel on the plane.

  3. Step 3:

    Use the rectanle to perform an Extrude Cut (use the Reverse Direction option)

  4. Step 4:

    Repeat these steps to cut the other plane.