The value of $\mathrm{\Delta }H-\mathrm{\Delta }U$ when 2 moles of solid benzoic acid undergoes combustion at 300 K is given by 

Here;

${\mathrm{C}}_6{\mathrm{H}}_5\mathrm{COOH}\left(s\right)+\frac{15}{2}{\mathrm{O}}_2\left(g\right)⟶7{\mathrm{CO}}_2\left(g\right)+3{\mathrm{H}}_2\mathrm{O}\left(l\right)$
In the this reaction the difference of change in enthalpy and change in internal energy,

$\begin{array}{l}\mathrm{\Delta }H-\mathrm{\Delta }U=\mathrm{\Delta }{n}_{g}RT=-\frac{1}{2}\times \frac{8.314\times 300}{1000}\\ =-1.247 \mathrm{kJ}\end{array}$

$\therefore$ Foe 2 moles $=-1.247\times 2=-2.494 \mathrm{kJ}$