The molar mass of stearic acid is 18*AC+36*AH+2*AO=18*12+36*1+16*2=284g/mol.
n=m/M=2.831/284=0.01 moles
C18H36O2+27O2-->18CO2+18H2O
we have 18*0.01=0.18 moles of CO2
18*0.01=0.18 moles of H2O
0.01*948=9.48kJ from stearic acid
0.18*393.5=70.83kJ from CO2
0.18*241.826=43.52kJ from H2O
9.48+70.83+43.52=123.83kJ
123.83*4.184=518.10kcal