A large spherical mass $M$ is fixed at one position and two identical point masses $m$ are kept on a line passing through the centre of M (see figure). The point masses are connected by a rigid massless rod of length $\ell$ and this assembly is free to move along the line connecting them. All three masses interact only through their mutual gravitational interaction. When the point mass nearer to M is at a distance $\mathrm{r}=3 \ell$ from M , the tension in the rod is zero for $m=k\left(\frac{M}{288}\right)$. The value of $k$ is