To find the number of moles of a substance, you need to divide the mass of the substance by its molar mass.
The molar mass of Rb2S is (2x85.468 + 32.06) g/mol = 212.996 g/mol
So, to find the number of moles of Rb2S in 368g, you would divide 368g by 212.996 g/mol :
moles = 368 g / 212.996 g/mol = 1.73 moles
There are 1.73 moles of Rb2S in 368g.