Here it is
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First adding H3O+ will hydrolyze it and convert it in ethylene tetracarboxylic acid which on heating will be converted back to anhydride as starting material.
DBO or Index of H or Degree of unsaturation is counted as number of H2 molecules needed to remove all pi bonds and rings.
DBO = number of pi bonds + rings 

We can find a double bond equivalent by calculating the index of hydrogen deficiency (IHD).  By doing so we can find whether and how many multiple bonds and rings are involved. IHD is also called the Degree of Unsaturation. The degree of unsaturation indicates the total number of pi bonds and rings within a molecule. Finding a degree of unsaturation makes it easier to figure out the molecular structure.You can easily find Degree of Unsaturation if a molecular formula is given. All you have to do is plug the number into the following formula.DOU = (2C+2+N-X- H) / 2. Here C is the number of carbons, N is the number of nitrogens, X is the number of halogens (F, Cl, Br, I) and H is the number of hydrogens.
Hope it helps.