Exothermic reaction discharges energy in forms of light, heat or sound, thus it is represented by negative flow of heat. In this equation:
NaOH (aq) + H2SO4 (aq) à H2O (l) + Na2SO4 (aq)
When the solution of Sodium hydroxide is added to the Sulphuric acid, water and Sodium sulfate is produced. To balance this, we put (2) two on Sodium hydroxide and (2) two on water.
2NaOH (aq) + H2SO4 (aq) à 2H2O (l) + Na2SO4 (aq)