The chemical formula of the compound calcium chlorate is
The subscript on each symbol indicates the number of atoms of that element in one molecule or formula unit. The subscript outside the parentheses tells you to multiply everything inside the parentheses by that number. So here's how many atoms of each element are in one molecule of calcium chlorate:
O: 2 x 3 = 6
In order to answer this question you need to be able to write the formula of calcium chlorate from its name. It's an ionic compound containing the ion Ca2+ and the polyatomic ion ClO3 -. The positive charge must cancel out the negative charge because compounds are neutral. The formula will need two chlorate ions at negative one each to have a total negative charge of -2. This balances the positive charge of +2 on the calcium ion.
The calcium ion's charge can be determined from it's group on the periodic table. Calcium is in Group 2, which has elements that always form +2 ions.
The clue that chlorate is a polyatomic ion is its ending in -ate rather than -ide. You can find its formula and charge on a table of polyatomic ions.