In this diagram, we have a reaction occurring and 3 letters that represent Reactants, Activation Energy and Products. To determine which is which, first, we will almost always have the Activation Energy in the middle of the reaction, this is how much energy is required for the reaction to occur, therefore Activation Energy is represented by letter B
To determine the total change in enthalpy, we need to subtract the sum of all the enthalpy from the products, which is letter C, and the sum of all the enthalpy from the reactants, which is letter A
ΔH = C - A
In this graph, we have the energy increasing and more energy on the products side, this means that an input of energy was required, this is the description of an Endothermic reaction, therefore the sign will be Positive