Define standard enthalpy of formation.

The standard enthalpies of formation of H_(2)O(l) and CO_(2)(g) are respectively -286" kJ mol"^(-1) and -394" kJ mol"^(-1) . If the standard heat of combustion of CH_(4)(g)" is "-891" kJ mol"^(-1) , then the standard enthalpy of formation of CH_(4)(g) is

a) What is meant by enthalpy of formation? b) What is the value of standard enthalpy of formation ( Delta_fH^o ) of an element?