H2 + O2 ---> H20

2, 1, 1
2, 2, 1
2, 1, 2
1, 2, 2

2 H2 + O2 => 2 H2O aka 2, 1, 2

Explanation:

H2 + O2 => H2O

On the left side there are currently 2 H's and 2 O's

On the right side there are currently 2 H's and 1 O

We have to match both sides to equal the same number of H's and O's

As previously stated, both sides have 2 H's without doing anything to them so let's ignore that and skip to the O2

One the left there are 2 O's and on the right there is 1 O

We cannot remove O's so in order to equal each other we must look at the right side

To get from 1 O to 2 O, we can multiply H2O by 2

This will give us 4 H's, and 2 O's

This makes the O's equivalent to each other

But now, the H's are imbalanced

The left side has 2 H's while the right side has 4 H's

To make the 2 H's go to 4 H's, we multiply the 2 H's by 2

This gives us 4 H's on both sides and now the equation is balanced