Use the data in the table given below to calculate an approximate δh∘ in kilojoules for the synthesis of hydrazine from ammonia: 2nh3(g)+cl2(g)⟶n2h4(g)+2hcl(g)

On the left (reactant) side you have 6 N-H bonds and 1 Cl-Cl bond 
On the right (product) side you have 4 N-H bonds, 1 N-N bond and 2 H-Cl bonds 

Add up the bond energies for each side (multiplying by the number of bonds) and calculate the difference for your ΔHº

While unproven, it uses facts to support part of a conclusion. it is still factual and has predictive power, however.