It is Ca(NO₃)₂ because NO₃ has a charge of -1, and Ca has a charge of 2+. And when writing an ionic compound, you take the charge of 1 and write it as the subscript of the other.
Now let's balance the equation and then find the net ionic equation. 2HNO₃(aq) +Ca(OH)₂(aq) --> 2H₂O(l) + Ca(NO₃)₂(aq)