Answer
The activation energy for the decomposition of $XY$ is $139\space kJ/(2\space mol)$.
Work Step by Step
The activation energy can be found by taking the activation energy ($167kJ/mol$) and subtracting the enthalpy of reaction ($28kJ/mol$). Since the chemical equation of the formation of $XY$ created $2\space mol$ of $XY$, the decomposition reaction also requires $2\space mol$ of $XY$, hence the per $2\space mol$ in the unit.
