9. As shown in the figure, the perimeter of hexagon $A B C D E F$ is 16 cm, all six angles are $120^{\circ}$, if $A B=B C=C D=3$ cm, then $E F=$ $\qquad$cm.
9. As shown in the figure, the perimeter of hexagon $A B C D E F$ is 16 cm, and each of the six angles is $120^{\circ}$. If $A B=B C=C D=3$ cm, then $E F=5$ cm.
【Solution】Solution: As shown in the figure, extend and reverse extend $A F, B C, D E$, intersecting at points $G, H, N$, respectively. Since each angle of hexagon $A B C D E F$ is $120^{\circ}$,
so $\angle G=\angle H=\angle N=60^{\circ}$
Therefore, $\triangle G H N, \triangle G A B, \triangle H C D, \triangle E F N$ are all equilateral triangles. $A B=B C=C D=3$ cm,
The side length of $\triangle G H N$ is
$$
\begin{array}{l}
3+3+3=9 \text { (cm) } \\
A N=9-3=6 \text { (cm) } \\
A N=A F+E F
\end{array}
$$
$D E=$ Perimeter of hexagon $A B C D E F - A B - B C - C D - (A F + E F)$
$$
=16-3-3-3-6
$$
$=1$ (cm)
$E F=E N=9-3-1=5$ (cm)
Answer: $E F=5$ cm.
Therefore, the answer is: 5.