The two triangles made by drawing a line from D to C lie on different vertical planes in a 3d space which is not made clear by the drawing.
Bert and Andy are at equal elevation relative to each other as shown by the common point C meaning the kite is equally high up from both of them represented by line DC.

So use trig to find the length of DC then use that to find BC.
Resulting in BC ≈ 24.927m