Heat of combustion (ΔH°c) is the energy released when a substance is burnt in oxygen.

The heat of formation (ΔH°f) is the energy released or absorbed when a substance is formed from its constituent elements.

The chemical equation is  H\(_2\) +  1/2 O\(_2\)  →  H\(_2\)O

Since the heat of formation of H\(_2\)  is zero (ΔHf(H\(_2\) ) = 0 kJ/mol), and the heat of combustion of H\(_2\)  is -285.8 kJ/mol, we can use the following relationship:

ΔHc(H\(_2\) ) = ΔHf( H\(_2\)O) - ΔHf(H\(_2\) )

Rearrange to solve for ΔHf( H\(_2\)O):

ΔHf( H\(_2\)O) = ΔHc(H\(_2\) ) + ΔHf(H\(_2\) )

= -285.8 kJ/mol + 0 kJ/mol

= -285.8 kJ/mol