I didn't do this with a pencil and pad but try this
Add equn 1 to equan 2 to the reverse of equan 3 and see if that won't give you the equation you want. For the dH values, change the sign of those you reverse.
Use Hess's law and the following equations to calculate ÄH for the reaction N2(g) + 2O2(g) N2O4(g).
2NO2(g)->N2O4(g) ÄH = –57.0 kJ/mol
N2(g)+O2(g)->2NO(g) ÄH = 180.6 kJ/mol
2NO2(g)->2NO(g)+O2(gÄH = 114.4 kJ/mol
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