In order to balance reactions, the number of each element must be the same on the left and right side. If I have 3 of element X on one side, I must have 3 of element X on the other side. That being said:
Zn + Fe(NO3)2 —> ZnNO3 + Fe
I have two NO3 groups on the left, so I will add a coefficient of 2 in front of the compound which has an NO3 group on the right side.
Zn + Fe(NO3)2 —> 2ZnNO3 + Fe
Since I now have 2 Zn’s on the right, and only 1 on the left, I will add a coefficient of 2 in front of Zn on the left.
2Zn + Fe(NO3)2 —> 2ZnNO3 + Fe
Lets check: Zn: 2 right, 2 left (correct) Fe: 1 right, 1 left (correct) NO3 groups: 2 right, 2 left
