Basically there should be the same number/amount of each element on both sides. For example, if you had MgO + Ca3N2 --> Mg3N2 + CaO, and the problem asked you to balance it, the correct answer would be: 3MgO + Ca3N2 --> Mg3N2 + 3CaO. You would put the numbers in the front of the compounds to make sure that oxygen, calcium, magnesium, and nitrogen would have the same amount as it did on the other side. I hope that makes sense. :)