G3.4 In Figure $1, A B C D$ is a rectangle, $F$ is the midpoint of $C D$ and $B E: E C=1: 3$. If the area of the rectangle $A B C D$ is $12 \mathrm{~cm}^{2}$ and the area of $B E F D$ is $R \mathrm{~cm}^{2}$, find the value of $R$.
Area of $\triangle B C D=6 \mathrm{~cm}^{2}$
Area of $\triangle C E F=\frac{3}{4} \cdot \frac{1}{2} \cdot 6 \mathrm{~cm}^{2}=\frac{9}{4} \mathrm{~cm}^{2}$
Area of $B E F D=\left(6-\frac{9}{4}\right) \mathrm{cm}^{2}=\frac{15}{4} \mathrm{~cm}^{2}$