Simple AS stoichiometry - HELP!!!

I did AS chemistry a couple of years ago (OCR), but have completely forgotten how to work through problems such as the one below. If anyone could work through that example for me, it would be much appreciated. xxx

Phosphorus may be determined analytically by precipitating it as MgNH4PO4, followed by ignition to Mg2P2O7. What weight of Mg2P2O7 will be obtained from a 2.558g sample of phosphate rock, which contains 86.52% Ca3(PO4)2?

Reply 1

Eau

Don't worry about intermediate precipitate. 1 mole of the P in rock produces 1 mole of P in the last product.

So work out the number of moles of P in rock. That would be the number of moles of ignition product. Then multiply by RMM(product).