I1.3 In the figure, $B D=b \mathrm{~cm}, D C=c \mathrm{~cm}$ and area of $\triangle A B D=\frac{1}{3} \times$ area of $\triangle A B C$, find the value of $c$.
Let the common height be $h \mathrm{~cm}$
$$
\begin{array}{l}
\frac{1}{2} B D \times h c m=\frac{1}{3} \cdot \frac{1}{2} B C \times h c m \\
2=\frac{1}{3}(2+c) \Rightarrow c=4
\end{array}
$$