[johnnyjohn993123]

ALL ARE IN gas Phase:

N₂ + O₂  2NO  ΔH= +180.7KJ

2NO +O₂  2NO₂  ΔH= -113.1KJ

2N₂O 2N₂+ O₂ΔH=-163.2KJ

USE HESS'S LAW TO CALCULATE  ΔH for the reaction
N₂O +NO₂  3NO

mY Attempt:

3/2 ( N₂ + O₂  2NO )

3/2 ( 2NO₂  2NO + O₂  )

1/2( 2N₂O 2N₂+ O₂ )

 I can't seem to get the same chemical equation as we're suppose to. Can any of you see my mistake/s and please do correct me. Its very confusing what should I subtract, for me to able to arrive at the same equation as before. 

[mjc123]

Let's call your given reactions A, B and C.
The simplest way to start is to see if there are any reagents that only occur once in A, B and C. (This is not always possible.)
NO₂ appears only once, on the RHS of B
N₂O appears only once, on the LHS of C.
These both appear on the LHS of your target equation. So let's try -1/2 B + 1/2 C:
NO₂ + N₂O  NO +1/2 O₂ + N₂ + 1/2O₂ = NO + N₂ + O₂
You can see that N₂ and O₂ are on the LHS of A, so add A:
NO + N₂ + O₂  NO + 2NO = 3NO
So your overall reaction is -1/2 B + 1/2 C + A.