Chemistry: The Central Science (13th Edition)

Ca(ClO$_{3}$)$_{2}$
Ca$^{2+}$ has a 2+ charge, and ClO$_{3}^{-}$ has a 1- charge. Therefore you need two ClO$_{3}^{-}$ to balance out one Ca$^{2+}$ to create a neutral compound, and the chemical formula must be Ca(ClO$_{3}$)$_{2}$.