you are calculating the total sum of energy released during combustion and absorbed when they form new bonds
so total energy to break oxygen (O2) bond is 119kcal and for C2H6 it is 99×6+83=677
total energy to form CO2 is 192×2 =384kcal and 111×2=222 kcal for H2O
total enthalpy change= -119 + (-677) + 384 + 222
=-190kcal
