Here's why:

* Nitrogen (N): On the left side, you have 1 nitrogen atom in NH3 (and since there are 4 NH3 molecules, it's 4 nitrogen atoms total). On the right side, you have 2 nitrogen atoms in N2.

* Hydrogen (H): On the left side, you have 3 hydrogen atoms in NH3 (and 12 total since there are 4 NH3). On the right side, you have 6 hydrogen atoms in H2.

Here's the balanced equation:

2 NH3 → N2 + 3 H2

Explanation:

* Nitrogen: To balance the nitrogen, we put a coefficient of 2 in front of NH3 on the left side.

* Hydrogen: This automatically balances the hydrogen, as there are now 6 hydrogen atoms on both sides.

This balanced equation shows that 2 molecules of ammonia (NH3) decompose to form 1 molecule of nitrogen gas (N2) and 3 molecules of hydrogen gas (H2).