Answer:
2H2 + O2 = 2H2O
Step-by-step explanation:
From the original equation, you first need to write each component separately. 
 
Left side: H = 2 ; O = 2 (number based on the subscript) 
Right side: H = 2 ; O = 1 (number based on the subscripts; no subscript means that the element is just 1) 
 
Notice that the number of H is already balance but the number of O is not. In order to balance the O, you need to multiply the element by 2, but you CANNOT do this by simply changing the subscript. 
 
Left side: H = 2 ; O = 2 
Right side: H = 2 x 2 = 4 ; O = 1 x 2 = 2 
 
Now, notice that the number of O is now balance (both are 2) but the number of H is not (since 
You also multiply the left side H by 2. Hence, 
 
Left side: H = 2 x 2 = 4 ; O = 2 
Right side: H = 2 x 2 = 4 ; O = 1 x 2 = 2 
 
The equation is now balanced.