Answer:
27
Explanation:
Name the midpoint of BC point M and the midpoint of CD point N.
Triangle ABM has the 1/4 the area of the rectangle, 18. Triangle ADN also has 1/4 the area of the rectangle, 18. Triangle CMN has 1/8 the area of the rectangle, 9. That is, of the rectangle area, ...
18 + 18 + 9 = 45
are not included in triangle AMN. So, the area of that triangle is ...
72 -45 = 27 . . . . units²
_____
Comment on the triangle area
The triangle created by joining the midpoint of the side of a rectangle with an opposite vertex will have the same dimension as the rectangle in one direction (L), and half the dimension of the rectangle in the other direction (W). Thus, where the rectangle area is LW, the triangle area is (1/2)(L)(1/2W) = (1/4)LW, 1/4 the area of the rectangle.
Similarly, if the midpoints of adjacent sides are joined to form a triangle, the area of that is (1/2)(1/2L)(1/2W) = (1/8)LW, or 1/8 the area of the rectangle.