Answer
The resonance structure should look like this:
Work Step by Step
1. Draw all lone pairs that are missing.
As we can see, the oxygen had lone pairs that were not drawed.
2. Identify the lone pair next to the pi bond:
They are circled in red in the image. "Next" mean that, there is exactly one bond between the lone pair and the pi bond.
3. Draw the arrows:
Follow the same pattern we have learned in the chapter.
(Bring down the lone pair to the bond in the middle, and kick up the pi bond to form a lone pair.)
4. Do the changes based on the arrows.
* Don't forget the formal charges!
