Example:

The dimensions are:
sheet of paper = 8.5" x 11"
circle = 15 cm
triangle length = 5 cm
To my eye, the triangle is drawn in the rule of thirds of the circle. To finish it off, there appears to be a black rectangle over the the top 2/3s of the circle/triangle as you see with the lines of them inverted. How would I go about creating this in Inkscape?
EDIT: OK! I think I am mostly there using just shape tools and rectangles as guides. How can I finish this with the larger inverted rectangle?