To go from kJ/mol to kJ/kg you have to use water's molar mass, which expresses the mass in grams of 1 mole of water.
Notice that you have 2 moles of water being formed by the reaction, which means that the enthalpy change of your reaction is actually -571.6 kJ. The standard enthalpy of formation per mole of water will be
Use the fact that 1 mole of water weighs 18.015 g, which means that
Rounded to four sig figs and expressed in kJ, the answer will be