What is the standard enthalpy of formation?

For a particular isomer of C8H18, the following reaction produces 5113.3 kJ of heat per mole of C8H18(g) consumed, under standard conditions.

C8H8 +25/2 O2(g) –> 8CO2(g) + 9H2O(g) delta H = – 5113.3 kj

What is the standard enthalpy of formation of this isomer of C8H18(g)?