Answer :
A chemical equation is balanced by changing or adding C. numbers in front of molecules, which are called coefficients.
For example:
O2 + 2 H2 ---> 2 H2O (number 2 is added to both sides in order to balance the equation)
For example:
O2 + 2 H2 ---> 2 H2O (number 2 is added to both sides in order to balance the equation)