Question

calculate the enthalpy for the reaction D + F = G + H using
G + C = A + B deltaH = 277
C + F = A delta H = 303
D = B + H delta H = -158

Answer 1

Reverse eqn 1 and add to the sum of eqn 2 and eqn 3. Cancel those substances that appear on both sides of the equation. Make sure the sum is the equation you want. Then change the sign for delta H for reversed equations and add to equations not reversed to get total delta H for the reaction. Post your work if you get stuck.