$27$
13. As shown in the figure, the diagonal $BD$ of rectangle $ABCD$ passes through the origin $O$, and the sides of the rectangle are parallel to the coordinate axes. The graph of the inverse proportion function $y=\frac{k}{x}$ intersects $BC$ and $CD$ at points $M$ and $N$, respectively. Given that point $A(-3,-3)$, and the area of $\triangle OMN$ is 3, then $k^{2}=$ . $\qquad$