In a chemical reaction, energy must be supplied to break the bonds in the reactants. Energy is released when new bonds form in the products. The energy needed to break a bond and the energy released when it forms are given by bond energies, usually in kJ/mol. In the exam the values you need are supplied.
The overall energy change is the difference between the two totals:
energy change = energy needed to break bonds − energy released when bonds are formed
If the energy released forming new bonds is greater than the energy needed to break the old bonds, the reaction is exothermic and the energy change is negative. If the energy needed to break bonds is greater than the energy released forming new bonds, the reaction is endothermic and the energy change is positive.
Example: H2 + Cl2 → 2HCl. The bond energies are H–H 436, Cl–Cl 243 and H–Cl 432 kJ/mol. Breaking bonds needs 436 + 243 = 679 kJ/mol. Forming two H–Cl bonds releases 2 × 432 = 864 kJ/mol. The energy change is 679 − 864 = −185 kJ/mol, so the reaction is exothermic.