Even with "not-armed" eye, it is clearly seen from the coordinates, that the polygon is a trapezoid with horizontal bases of the lengths of 15-9 = 6 and 18-6 = 12 units. The altitude is 27-12 = 15 units. The area of a trapezoid is the product of the half of the sum of its bases (which is (6+12)/2 = 9 units) and the altitude, so the area is 15*9 = 135 square units.