You still have to fix the signs on the ΔH values for those two reactions. That was my excuse to review the meanings of endothermic and exothermic reactions with you, since these terms do apply here. 
An endothermic reaction absorbs heat from the surrounding solvent in the calorimeter, and from the thermometer too. The final thermometer temperature will be less than the initial temperature in an endothermic reaction. An exothermic reaction produces heat which is absorbed by the surroundings and thermometer. The final temperature is higher than the initial temperature after the reaction. So what you need to do is go back and check the temperature changes for these two reactions from your data:
If Tf < Ti, the reaction is endothermic and ΔH is positive.
If Tf > Ti, the reaction is exothermic reaction and ΔH is negative.
The sign of ΔH tell us whether the system, which is the reaction, gained energy in the form of heat or lost energy in the form of heat. The definition of ΔH is the change in heat energy in a reaction or process under constant pressure conditions, which is what we have in the open room under a constant atmospheric pressure. If the system loses heat energy (exothermic process) it's like we made a "withdrawal" (negative change) of heat from the system. If the system gains heat energy (endothermic) it's like we made a "deposit" (positive change) of heat energy to the system.
If you reverse a reaction, change the sign of its ΔH. You will need to use all four reactions in message 2 to get the desired reaction (also in message 2). Substances have to be on opposite sides in order to cancel. In your last message, the NH4Cls do not cancel because they are on the same side. You will have to reverse one of these reactions before adding them together.
So first, be sure you have the correct signs for the ΔH values for your two experiment reactions, and then you will be set to do this last part, using the four reactions with known ΔHs to determine ΔH of a new reaction.
Steve
P.S. Thermodynamics is a challenging subject! Do you have a textbook to use with this online course? Even with a textbook, learning thermodynamics can be a slow process. Keep at it.
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