Answer: There is two (PO4) in the formula
Explanation:
Just be aware of the difference between numbers to the right and left: 5Mg3 means there are a total of 5 Mg (if 3 is a number in the bottom left) the 3 in (PO4) means there are 3 PO4 but because of the 2 it could mean there are 6.
Depends if the elements are separated
