The heat of combustion of solid benzoic acid at constant volume is \( -321.30 \, \text{kJ} \) at \( 27^\circ \mathrm{C} \). The heat of combustion at constant pressure is \( (-321.30 - xR) \, \text{kJ} \), the value of \( x \) is \( \_\_\_\_\_\_\_\_ \).