The representation of a chemical reaction in the form of symbols (substances) is known as a chemical equation. A chemical equation consists of reactants, products and an arrow showing the direction of the reaction. The equation in which the number of atoms of all the molecules is equal on both sides of the equation is known as the balanced chemical equation.
There are 2 N on the left and only one on the right. A coefficient of 2 for NH3 will satisfy the mass requirements.
N2 + 3H2 → 2 NH3