Answer
$-296.8\ kJ/mol$
Work Step by Step
From the definition of heat capacity:
$(923\ J/K+815\ g\times 4.184\ J/g.K ) \times(26.72-21.25:)\ K=23.7\ kJ$
Negative value because it's an exothermic process.
Number of moles of $S_8$:
$2.56\ g\div 256.48\ g/mol=0.00998\ mol$
From stoichiometry:
$0.0799\ mol\ SO_2$
Enthalpy of combustion:
$-23.7\ kJ/0.0799\ mol=-296.8\ kJ/mol$
