First you gotta figure out which phosphate you're using so you have the correct mass. Because you are reacting with +2 Ions you'll be needing disodium phosphate (Na2HPO4 -- FM = 141.96g/mol)
Next, find the moles of your elemental calcium / magnesium being reacted with:
.055 mol/L Ca+2 x 1.5L = .0825. Now you have .0825 moles Ca2+ and it's a 1:1 mol to mol ratio between Ca and phosphate, so .0825 mol x 141.96 g/mol = 11.7117g Na2HPO4 required!
Using this formula find the next one on your own! (HINT: this reaction is also 1:1 ratio)