$87.75\mathrm{kJ}{\mathrm{mol}}^{-}$ energy is absorbed to convert $2.0{\mathrm{gO}}^{-}\left(\mathrm{g}\right)\text{ to }{\mathrm{O}}^{2-}\left(\mathrm{g}\right)$. The second electron gain enthalpy of oxygen will be 

${\mathrm{O}}^{-}(\mathrm{g}.)+{\mathrm{e}}^{-}+\mathrm{EA}⟶{\mathrm{O}}^{2-}\left(\mathrm{g}\right)$

$16\mathrm{g}. \mathrm{xkJ}$

Electron gain enthalpy for $2\mathrm{g}\cdot {\mathrm{O}}^{-}(\mathrm{g}.)=87.75\mathrm{kJ}$

Electron gain enthalpy for

 $\begin{array}{r}16\mathrm{g}\cdot {\mathrm{O}}^{-}(\mathrm{g}.)=\frac{87.75}{2}\times 16\\ =702\mathrm{kJ}\end{array}$