Call the vertices A, B, C & D Arrange them: A B C D A 3 6 9 6 3 3 6 3 0 3

= 3*6 + 6*3 + 9*0 + 6*3 = 54 Add the diagonal products starting with the lower left 3*6 + 6*9 + 3*6 + 0*3 = 90 ---------- The area = 1/2 the absolute value of the difference = (1/2)*36 Area = 18