c,
$\Delta$ HKB $\backsim$ $\Delta$ AHB (g.g)
$\Rightarrow \dfrac{HK}{AH}=\dfrac{BK}{BH}=\dfrac{BH}{AB}$
$\Leftrightarrow \dfrac{HK}{7,2}=\dfrac{BK}{5,4}=\dfrac{5,4}{9}$
$\Rightarrow HK=4,32; BK=3,24$
$\Rightarrow AK= 9-3,24=5,76$
Diện tích hình thang AKHC (KH // AE):
$S=\dfrac{(4,32+12).5,76}{2}= 47,0016(cm^2)$