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Constitutional isomers are compounds that have the same molecular formula and different connectivity. To determine whether two molecules are constitutional isomers, just count the number of each atom in both molecules and see how the atoms are arranged. In this question, the molecular formula of both the compounds is $C_2H_4 Cl_2$. However, on the left one the Chlorines are on the different side and on the right one the Chlorines are on the same side.