Balancing chemical reactions is based on the Law of conservation of mass, which tells us that mass cannot be created nor destroyed, only be converted.
Therefore in a chemical reaction, the masses at both sides should be the same.
Let's take your reaction:
We first see that on the left side of the arrow, we have one carbon atom, and on the right side we have two carbon atoms. This directly violates the law of conservation of mass, since the mass of the product (the right side of the arrow) is twice as big as the mass on the left side of the arrow.
To balance the reaction, we must put a 2 as a coefficient in front of the C at the left side. We then obtain:
The masses are now equal on both sides of the arrow!
However, the 2 is not placed the same position, The lower 2 from
The two in
In these questions you could use:
Balancing chemical reactions can only be done by placing coefficients in front of the molecule, not by changing the molecule