6. The length of side $A D$ of the inscribed quadrilateral $A B C D$ is 3. Point $M$ divides this side in the ratio $A M: M D=1: 2$, and the lines $M C$ and $M B$ are parallel to the sides $A B$ and $C D$ respectively. Find the length of side $B C$ of the quadrilateral.
Answer: $B C=\sqrt{2}$.

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