Balancing the Chemical Equation

I need help balancing this chemical equation. Here it is.

(NH[sub]4)[sub]3PO[sub]4 + Pb(NO[sub]3)[sub]4 ----> Pb[sub]3(PO[sub]4)[sub]4 + NH[sub]4NO[sub]3.

[sub]= subscript.

Please help, i've been working at this one problem for 20 minutes straight. And can you explain it please?

4 (NH4)3PO4 + 3 Pb(NO3)4 → Pb3(PO4)4 + 12 NH4NO3