For the reaction ${\mathrm{X}}_2{\mathrm{O}}_{4\left(\mathrm{l}\right)}\to 2{\mathrm{XO}}_{2\left(\mathrm{g}\right)}$
Given $\mathrm{\Delta H}=3.3\mathrm{kcal},\mathrm{\Delta S}=20\mathrm{cal}{\mathrm{K}}^{-1}$ at 300 K. Then $\mathrm{\Delta G}$ of the reaction is 

$\mathrm{\Delta G}=\mathrm{\Delta H}-\mathrm{T\Delta S}$
Given, $\mathrm{\Delta S}=20\times {10}^{-3}\mathrm{kcal}{\mathrm{K}}^{-1}$
On putting the values of  $\mathrm{\Delta H}\mathrm{and}\mathrm{\Delta S}$ in the equation,
we get $\mathrm{\Delta G}=3.3-300\times 20\times {10}^{-3}$
$=3.3-6\times {10}^3\times {10}^{-3}=-2.7\mathrm{kcal}$