15. In $\triangle A B C$, the sides opposite to angles $A , B , C$ are $a , b , c$ respectively. If $b = 6 , a = 2 c , B = \frac { \pi } { 3 }$, then the area of $\triangle A B C$ is $\_\_\_\_$ .
0.18
15. In $\triangle A B C$, the sides opposite to angles $A , B , C$ are $a , b , c$ respectively. If $b = 6 , a = 2 c , B = \frac { \pi } { 3 }$, then the area of $\triangle A B C$ is $\_\_\_\_$ .