It should be available in books. I found a reported value online for the enthalpy of combustion of hexane but it might well not be solid hexane (-3873 kJ mol-1). Off a pure guess based on hexane's standard state, this probably refers to liquid hexane, so all you have to do is add the enthalpy change for Hexane (s)
Hexane (l) and you'll get the enthalpy change for combustion of liquid hexane. Then you simply divide by M
r to get specific enthalpy.