Answer:
First choice: 2
Explanation:
There are 2 phosphorous (P) in the substance.
Ignore the strontium (Sr3) part because you are looking to isolate the P from (PO4)2.
Break the chemical equation apart to get 1 Phosphorous atom, and 4 Oxygen atoms.
Now, multiple 1 by 2 because that are 2 phosphate to get 2 phosphorous atoms.
