To begin with, you need a solubility chart present. This would inform you that PO4 + Ca forms an insoluble product.
Now write a balanced chemical equation for the reaction taking place.
2K3PO4 + 3Ca --- Ca3(PO4)2 + 6K+
0.803 mol K3PO4 creates 0.803 mol (PO4)3-
Using the chemical equation above, you know that for every 2 moles of K3PO4, 3 moles of Ca are consumed.
0.803 mol K3PO4 x (3 mol Ca / 2 mol K3PO4) = 1.20 mol Ca
To find grams, just use the molar mass of calcium, 40.08 g / mol to convert the molar amount to grams.
1.20 mol Ca x (40.08 g /mol) = 48.3 g Ca