A student dissolves 22.4 g of sodium phosphate to prepare a 2.98 L solution. Another student then dilutes this solution to 4.90 L. If sodium phosphate completely dissociates, determine the number of mols of each ion in the final solution.

The 2.98 L is a false alarm and just extra data. Ignore it. So is the 4.90L. Ignore it too.

You have how many mols Na3PO4. That's mols = grams/molar mass = ?
So mols Na^+ = 3x mols Na3PO4.
and mols PO4^3- = mols Na3PO4.

I know students don't like to ignore data (it's there so I must need it) but trust me, both of these volumes are red herrings. mols = mols = mols, no matter how much water you add to them. If the question asks for molarity (M), that's a different matter.