Marcia is given an incomplete chemical equation that includes the number of nitrogen atoms present in the products of the reaction. How can she conclude that the reaction started with three nitrogen atoms?

There would have to be three nitrogen atoms in the products. The law of conservation of matter states that the amount of substance before a reaction occurs  should be the same as the amount of substance after the reaction.

Explanation: This is the EXACT sample answer from the test, just reword it if you want. ^