Answer
$+103.61\ kJ/mol$
Work Step by Step
Enthalpies of formation (kJ/mol):
$H_2O(l):-285.83, CO_2(g):393.509$
Enthalpy of combustion:
$-4395.0=4\times-285.83+8\times-393.509-x$
$x=+103.61\ kJ/mol$, which is the enthalpy of formation of styrene.