$\mathrm{A}_3 \mathrm{~B}_2$ is a sparingly soluble salt of molar mass $\mathrm{M}\left(\mathrm{g} \mathrm{mol}^{-1}\right)$ and solubility $\times \mathrm{g} \mathrm{L}^{-1}$. The solubility product satisfies $K_{s p}=a\left(\frac{x}{M}\right)^5$. The value of $a$ is $\_\_\_\_$ . (Integer answer)