作AE//CD交BC于E,作EF//AD交CD于F
则AEFD是平行四边形,且∠CFE=∠D=90
EF=AD=2√3
∠C=360-(∠A+∠B+∠D)=45
CF=EF=2√3
CE=√2*CF=√2*2√3=2√6
BE=BC-CE=6-2√6
易证∠BAE=45
所以,AB=BE=6-2√6
AE=√2BE=√2(6-2√6)=6√2-4√3
四边形ABCD的面积=S△ABE+SAEFD+S△CEF
=AB*BE/2+AE*AD+EF*CF/2
=(6-2√6)^2/2+(6√2-4√3)*2√3+(2√3)^2/2
=(60-24√6)/2+(12√6-24)+12/2
=30-12√6+12√6-24+6
=12