Balance the number of each element on both sides of the chemical reaction. We may start off with Carbon. Multiply the compound CO2 by 2. By this the number of oxygen on the right hand side of the chemical reaction is now 5. Multiply the O2 by 5/2. The chemical reaction becomes
C2H2 + (5/2)O2 ----> 2CO2 + H2O
To eliminate the fraction, multiple the whole chemical reaction by 2 to give the final answer of,
2C2H2 + 5O2 ---> 4CO2 + 2H2O