请解答这道数学题---**Problem 18**
**Question Stem:**
如图，在正方形 ABCD 中，E、F 是对角线 BD 的三等分点，延长 AE 交 BC 于 G，延长 GF 交 AD 于 H，已知 △DHF 面积是 9 平方厘米，则正方形 ABCD 面积是（ ）平方厘米。
**Textual Information (extracted from image):**
18、 如图，在正方形 ABCD 中，E、F 是对角线 BD 的三等分点，延长 AE 交 BC 于 G，延长 GF 交 AD 于 H，已知 △DHF 面积是 9 平方厘米，则正方形 ABCD 面积是（ ）平方厘米。
**Chart/Diagram Description:**
* **Type:** Geometric figure.
* **Main Elements:**
* A square ABCD is shown. Vertices are labeled A, B, C, D in counterclockwise order starting from the bottom left.
* The diagonal BD is drawn.
* Points E and F are located on the diagonal BD. E is closer to B, and F is closer to D, indicating they are trisection points.
* A line segment AE is drawn and extended to intersect side BC at point G.
* A line segment GF is drawn and extended to intersect side AD at point H.
* Points E, F, G, H are labeled.
* Triangle DHF is formed by vertices D, H, and F.
**Missing Information:**
* Options are not provided in the image. The question asks for the area of the square and leaves a blank space in parentheses.