The topless triangle is called a trapazoid. In the pattern you are showing, it's sort of like a flying goose unit with the top trimmed. An easy way to do this is measure the finished size of both the trapazoid and the wings add 1/2 inch to both length and width for the seam allowance. Cut two squares for the wings. Put them on the ends right sides together, sew on the diagonal and trim. Press open and you have the shape you are looking for.