Calculating change in enthalpy of combustion of 1.12g of hexane
I can't figure out exactly what you are trying to calculate, but if you want to know how much energy is created when burning 1.15 g of hexane by measuring an temperature increase of water, then this is the right way:

1.) 0.2 kg Water was icreased by 24 °C:
U = 0.2 kg * 4187 J/(kgK) * 24 K = 20,100 J = 20.100 kJ 

2.) n(Hexane) = 1.15 g / (86.18 g/mol) = 0.013 mol 

3.) 1.15 g caused the temperature increase of 24 °C:
20.100 kJ / 0.013 mol = 1,510 kJ/mol

So you will get 1.510 MJ out of burning 1 mol (86.18 g) hexane.