This is a simple stoichiometric calculation problem. The relevant stoichiometric calculations you should know are the following:
1 mol Ca₃(PO₄)₂ : 2 mol PO₄
1 mol Na₃PO₄ : 1 mol PO₄
Also the molar mass of Ca₃(PO₄)₂ is 310.18 g/mol.
The solution is as follows:
0.250 mol Na₃PO₄/L * 1 L/1000 mL * 94.2 mL * 1 mol PO₄/1 mol Na₃PO₄ * 1 mol Ca₃(PO₄)₂/2 mol PO₄ * 310.18 g/mol = 3.65 g Ca₃(PO₄)₂
