plus add two electrons to the right O. Add a 2- charge to the ion. This is ONE resonance form. You can draw the other two by moving the double bond from the central O (where I have it) to the top or to the bottom O.
Check to see that there are 24 electrons.
You must add two electrons to the top O, two to the right O, and two to the bottom O. I count 18 on what I drew, and it will make 24 if you had those 3 sets of two each.