How many of each atom are in #Pb_3(PO_4)_4#?
lead - 3
phosphorus - 4
oxygen - 16
Pb is lead, there is a small 3 after this so we only have three of them.
The use of brackets means everything in side the brackets has to be multiplied by the small number outside it.
Thus P (inside the brackets) needs to be multipled by the number outside - so overall we have 4
Oxygen inside the brackets has a small 4, but then we need to multiply that by the number outside the brackets 4. 4x4=16