The easiest way to balance any combustion equation is to first write the general form. In a combustion reaction the products are always carbon dioxide (CO2) and water (HOH) and the reactants are the organic material plus oxygen (O2).
C6H6 + O2 --> CO2 + HOH
Now start by balancing the carbon atoms:
six carbons on the left so you need 6 on the right.
C6H6 + O2 ---> 6 CO2 + HOH
Next balance the hydrogen atoms:
6 on the left, so you need 6 on the right.
C6H6 + O2 --> 6 CO2 + 3 HOH
Now count the number of oxygen on the right = 12 + 3 = 15
divide the number on the right by 2 to get the number needed on the left. 15/2 = 7.5
C6H6 + 7.5 O2 --> 6 CO2 + 3 HOH
But the rule is that coefficients have to be whole numbers. So multiple both sides by 2 to get the final balanced equation.
2 C6H6 + 15 O2 --> 12 CO2 + 6 HOH
To be sure, count the number of each element on each side:
On the left you have 12 C, 12 H, 30 O
On the right you have 12 C, 12 H, 30 O so it is balanced.