Why f is multiplied in solubility of salt constant I understand till we get “f” And also why f is directly proportional to PH. But I don’t understand why f is multiplied in solubility product with solubility of salt of weak acid.

(Sorry for poor english) (couldn’t find link of pdf of this book online)
Notice that $f$ is equal to the maximum concentration of $\ce{M}$, which is unchanged as part of the acid-base equilibrium. On the other hand, the pH changes the concentration of the counterion in proportion to $f$. Therefore, the second term in the solubility product is scaled accordingly.