Answer
The resonance forms are shown in the picture below.
Work Step by Step
The electrons involved in the bond between carbon 3 and carbon 4 from the left, delocalize between carbon 2 and 3 to satisfy the positive charge on carbon 2. Then the electrons in the bond between carbon 5 and 6, delocalize between carbon 4 and 5, to satisfy the positive charge on carbon 6.
