Hanoi Open Mathematics Competition 2012 Senior Question 3

In trapezoid \(ABCD\), the bases are \(BC=3\text{ cm}\), \(DA=6\text{ cm}\), with \(AD\parallel BC\). The segment \(EF\), where \(E\in AB,F\in CD,EF\parallel AD\), passes through \(M=AC\cap BD\). Find its length.