The process of solving this problem really needs Pythagoras' law and similar triangles's knowledge, which primary school students do not have. My solution process is for reference only.

From the topic "CA = 3, CB = 4 in the known right triangle ABC", we can know that AB=5 (Pythagorean law) and AB=AD+DB, AD = DB = 2.5. According to the fact that the triangle BDE is similar to the triangle BCA, we can use BD: BC = ED: CA to find DE = 2.5/4x3 =1.85. ,

Quadrilateral ACDE area = triangular area ABC- triangular bed area

= 3 X 4 / 2 — 2.5 X 1.875 / 2

= 6 — 2.34375

= 3.65625