What is the net ionic equation for calcuim ion and ammonium oxalate?

Answer 1

Look for a precipitate, a gas, or a weak electrolyte. In this case, CaC2O4 is insoluble. Here is a simplified list of the solubility rules.
http://www.files.chem.vt.edu/RVGS/ACT/notes/solubility_rules.html

Ca^+2(aq) + C2O4^-2(aq) ==> CaC2O4(s)