Please help, I am struggling to understand the counting of atoms

Check Box 1 and Box 3

Explanation: Formula 1 has (NO3)2 meaning it has 6 oxygen atoms (2x6) while formula 2 has CO3 with only 3 oxygen atoms.

Its essentially the same thing with box 3 with there being 8 hydrogen in formula 2 (NH4)2 and only 4 in formula 5 NH4 by itself

answer : third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol

explanation :

let us write down the given equations

equation 1 :

equation 2 :

equation 3 :

let us use hess's law to find out which 2 equations can be added to get the third equation.

let us reverse equation 2 and multiply it by 2. the new equation that we get is,

equation 4 :

let us add equation 1 & 4

the above equation is similar to equation 3 but has different value for dhrxn.

therefore third trial had the contaminated sample and the correct value for dhrxn should be 1000 kj/mol