Question

write ionic charge of anion of potassium carbonate..?

Answer 1

Using pretty basic nomenclature and knowledge of the formal charge of polyatomic ions. The symbol for Potassium is K, the chemical formula for Carbonate is CO3. The formal charge of Potassium is +1 and the formal charge of Carbonate is -2.

Answer 2

 Potassium (K) forms an ion of charge +1, and the carbonate ion (CO3) carries a -2 charge. To balance the -2 charge, you need 2 +1 charges 
-2 +1 +1 = 0 
Therefore you need 2 Potassium ions. 
So the formula is, obviously, K2CO3