Hello, Sophia,
The place to start is to determine the molar mass of Sn3(PO4)2. I'll assume you can calculate this based on adding the atomic masses of the individual elements to generate the total molecular weight of the molecule, which is also it's molar mass, g/mole.
I find 546.07 g/mole.
Convert 3.5 grams into moles by dividing it by the molar mass:
3.5g/546.1g/mole =0.006409 mole
One mole = 6.023x1023 molecules, in this cse. (A mole can be anything, atoms, molecules, bananas, pencils, ants, etc. - it is a term used as a unit of measure, just as in "a dozen" of anything).
So we have 0.00641 moles*(6.023x1023 molecules/mole) or 3.87x1024 molecules of Sn3(PO4)2
But note that each molecule has 3 Sn (tin) atoms. That means we need to multiply by 3, which brings us to 1.16x1025 tin(II) ions.
I hope this helps,
Bob