What is the relationship between stability and instability constant?

On the basis of stability of complex ion in the solution, complexes may be of two types, perfect and imperfect complexes. The stability depends upon the extent of dissociation which in turn depends upon the strength of metal-ligand bond. The stability of complex also depends upon charge on central metal  atom, basic nature of ligand, chelation, and nature of metal ion and ligand according to $\mathrm{HSAB}$ principle.

Which complex is most stable ?

For the complex ${\mathrm{ML}}_2,$ stepwise formation constants for $\mathrm{M}+\mathrm{L}\rightleftharpoons \mathrm{ML}$ and $\mathrm{ML}+\mathrm{L}\rightleftharpoons {\mathrm{ML}}_2$ are $4$ and $3$, respectively. Hence, the overall stability constant for $\mathrm{M}+2\mathrm{L}\rightleftharpoons {\mathrm{ML}}_2$ is

A blue colour complex is obtained in the analysis of ${\mathrm{Fe}}^{+3}$ having formula ${\mathrm{Fe}}_4{\left[\mathrm{Fe}(\mathrm{CN}{)}_6\right]}_3$.

Let $\mathrm{a}=$oxidation number of iron in the coordination sphere, $\mathrm{b}=$ no. of secondary valencies of central iron ion. $\mathrm{c}=$ Effective atomic number of iron in the coordination sphere.

Then find the value of $(\mathrm{c}+\mathrm{a}-2\mathrm{b}).$