What is the total number of atoms in the

formula Ca (NO3) 2?

(1) 7 (2) 2 (3) 3 (4) 9

The answer is 9 but I don't understand why, Can someone explain?

There is one calcium atom. Let's start there.

NO3 on its own will have 4 atoms; one nitrogen and 3 oxygens (remember, subscripts affect only the atom directly left). Since there are 2 NO3 compounds, there are 8 total atoms in the NO3s (4x2=8). Those 8 plus the calcium atom give you the 9 atoms.