The enthalpy of formation of water is -270kj/mol
H2 +1/2 O2 ------------>H2O ,

H =-270
The enthalpy of neutralisation of strong base and acid is -57kj/mol
H2O------------>H+ + OH- ,

H=57
enthalpy of formation is given by
Adding 1 and 2,we get
deltaH= -270+57 Kj/mol
=-213Kj/mol