Try drawing it. Plot a point in the middle of three of the sides. It cuts the rectangle in half. The area that remains would be 18 square units. Now draw a line from the middle of the base of the triangle (the line cutting the rectangle in half) up to the point above it. You have now cut the remaining rectangle part into two smaller rectangles. Each of these rectangles is cut in half by a diagonal line, which is a side of the triangle. I think this would mean that the triangle is taking up half of the area that is remaining, which would be 9 square units.