Trigonal planar, with 2 formal negative charges on the oxygen atoms.
There are #4+(3xx6)+2=24" electrons"# to distribute around 4 centres. A reasonable resonance isomer is #(O=)C(-O)_2^-#. Since there are three bonding pairs of electrons around the central carbon, trigonal planar geometry is indicated. Clearly, by resonance, all the #C-O# bonds are equivalent.