In carbonate ion Formal charge on doubly bonded oxygen is X and formal charge on Carbon atom is Y. Value of X and Y are respectively 

In carbonate ion doubly bonded Oxygen is having 2 lone pair electrons and 2 bond pair electrons.Formal charge  = $6-4-\frac{4}{2}=6-4-2=0$

Around Carbon atom 4 bonds are present.Formal charge  =  $4-0-\frac{8}{2}=0$