The equation for the reaction is
#"A + 2B" → "D" + "E"#
This is our target equation.
We must create it from equations (1) and (2).
(1) #color(white)(l)"A" + "2B" → "C"; Δ_"rxn"H = "+22.1 kJ"#
(2) #"C" → "D" + "E"; color(white)(ll) Δ_"rxn"H =color(white)(l) "-74.1 kJ"#
We start with Equation (1) because it contains #"A"#.
Then we add Equation (2) to eliminate compound #"C"# that is not in our target equation:
(1): #color(white)(m)"A" + "2B" → color(red)(cancel(color(black)("C"))) color(white)(mmll)Δ_"rxn"H = "+22.1 kJ"#
+(2): #color(red)(cancel(color(black)("C"))) color(white)(mml)→ "D" + "E" color(white)(m)Δ_"rxn"H = color(white)(l)"-74.1 kJ"#
#color(white)(mml)stackrel(————————)("A" + "2B" → "D" + "E")color(white)(ml) Δ_"rxn"H = stackrel(————)("-52.0 kJ")#
