i think it's Ca3(PO4)2.
The nonmetal atoms are P(phosphorus) and O (oxygen), basically we have 1 Phosphorus and 4 Oxygen but since the PO4 (phosphate) has a subscript 2, it means that we have to double everything inside the parenthesis.
So now we have
total # of nonmetal atoms = (1 Phosphorus x 2) + (4 Oxygen x 2)
= 2 Phosphorus + 8 Oxygen
= 10 nonmetal atoms
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