∴DE‖AB

∴△CED∽△CAB

∴S△CED/S△CAB=(0.5)？

∴S△DEC=6 cm2

You can't use congruence because you didn't say ∠ cab = 90,

∴△CDE is not completely equal to△△△ ade.

The people downstairs (the answer should be upstairs after revision) use simple bottom and high methods.