Answer
The resonance forms are shown in the picture below.
Work Step by Step
The electrons in the double bond of the first structure on the left, delocalize over the bond next to it, leaving the other C with two electrons and a negative charge. Then the electrons in the double bond between the carbon and the oxygen, form a double bond, leaving the The electrons on carbon delocalize and form a second double and the electrons between C-O bond are delocalized to form negative charge on Oxygen.
