Explain why a common ion lowers the solubility of an ionic compound

From Le Chatelier's principle, that happens to the solubility of a slightly soluble ionic compound when a common ion is added and The solubility of a slightly soluble ionic compound decreases. MX ---> M+ + X-

If you now have some other ionic compound such as QX, this will dissociate to Q+ + X-

Here, the common ion is X. This will push the reaction of MX ---> M+ X- back to the left because of adding more X-. This results in decrease in solubility.

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.