Note for problem 40 that the solubility of benzoic acid is the number of grams of solid benzoic acid that will dissolve in 100 mL of solution; or the number of moles of solid benzoic acid that will dissolve in 1 liter of solution.  The number of moles of solid benzoic acid that are dissolved includes the number of moles of C6H5COOH PLUS the number of moles of C6H5COO-.  The way I have set up the problem, that's what 'z' is.