Na₃PO₄(aq)+AlBr₃(aq)→3NaBr(aq)+AlPO₄(s)
This is a double replacement reaction. You can tell that there has to be 3NaBr molecules since there are 3 Na atoms in sodium phosphate and 3 Br atoms in aluminum bromide. We also know that Na has an oxidation number of +1 which means PO₄ needs to have an oxidation number of -3 while Br has an oxidation number of -1 which means Al has an oxidation number of +3. That means that Al³⁺ and PO₄³⁻ can from AlPO₄
I hope this helps. Let me know if anything is unclear.