6. (5 points) As shown in the figure, quadrilateral $ABCD$ is a square, $ABGF$ and $FGCD$ are both rectangles, point $E$ is on $AB$, and $EC$ intersects $FG$ at point $M$. If $AB=6$, and the area of $\triangle ECF$ is 12, then the area of $\triangle BCM$ is $\qquad$