The incomplete Lewis representation of  $C{O}_3^{2-}$ is given below. The formal charge on atoms marked as a.b and c, respectively, are

The Lewis structure of  $C{O}_3^{2-}$  is 
 

Formal charge (F.C.) = Valence e- - lone pair e-  - $\frac{1}{2}$ shared e-
F.C. on   $C_{\left(a\right)}=4-0-\frac{1}{2}\left(8\right)=0$
F.C. on   ${\text{O}}_{\left(b\right)}=6-4-\frac{1}{2}\left(4\right)=0$
F.C. on ${\text{O}}_{\left(c\right)}=6-6-\frac{1}{2}\left(2\right)=-1$