In this question, we have 5.5 moles of CaS, and in order to find the number of particles present in this amount of CaS, we need to use the Avogadro's constant number, which is 6.022*10^23, this number represents the amount of molecules or particles present in 1 mol, therefore:
1 mol = 6.022*10^23
5.5 moles = x particles
x = 3.31*10^24 particles