Hanoi Open Mathematics Competition 2010 Junior Question 9

Given a triangle $ABC$, let $D,M,N$ belong to $BC,AB,AC$, respectively. Suppose that $MD\parallel AC$ and $ND\parallel AB$. If $S_{\triangle BMD}=9\text{ cm}^2$ and $S_{\triangle DNC}=25\text{ cm}^2$, compute $S_{\triangle AMN}$.