#2CO + O_2 = 2CO_2#

#DeltaH"of reaction" = DeltaH_f^@ of "product(s)" - DeltaH_f^@ of "reactants"#

The product in this reaction is CO

#DeltaH_f^@# of CO is -253.7KJ/mol

The reactants in this reaction are #O_2# and #CO_2#

But the #DeltaH_f^@# of #O_2# is not there

The #DeltaH_f^@# of all elements is 0kJ/mol

So the #DeltaH_f^@# of #O_2# is

0kJ/mol

The combined #DeltaH_f^@#of products should be multiplied by their coefficients .

-253.7KJ/mol *2 + 0kJ/mol*1 = -507.4kJ/mol

#DeltaH_f^@# of products

#CO_2# = -314.3kJ/mol

And remember also multiply by the coefficient

-314.3kJ/mol* 2 = -628.6

Plug the values in the equation

#DeltaH"of reaction" = DeltaH_f^@ of "product(s)" - DeltaH_f^@ of "reactants"#

#"-628.6kJ/mol - (-507.4kJ/mol)"#

#DeltaH"of reaction"= "-121.2kJ/mol"#