有數學唔識

Because
△ABC~△DAC

CD/CA= CA/CB
CD/4 cm= 4/6
CD= 8/3 cm

AB/DA=CB/CA
AB/2 cm= 6/4
AB= 3 cm

You can use cosine law to calculate ㄥBAC and ㄥCAD
ㄥBAD depends how the figure is drawn (I hope you have the figure from your homework),
Either ㄥBAD= ㄥBAC+ㄥCAD or ㄥBAD= ㄥBAC-ㄥCAD

Cosine Law:
http://www.cut-the-knot.org/pythagoras/cosine.shtml

你要講埋個2個三角形係點? 分開定重疊~? 形容多D~!