This is a stoichiometry problem, where we have an initial amount of reactant and we need to find out how much of the product will we end up with, in order to do that we need to:
1. Set up the properly balanced equation, which the question already provided us
2 H2 + O2 -> 2 H2O
2. See how many moles of reactant there are in the given amount of grams, using the molar mass of H2, 2.016g/mol
2.016g = 1 mol
12.1g = x moles
x = 6 moles of H2
3. Check the molar ratio between the two compounds, which we can identify in the reaction, 2:2, so if we have 2 moles of H2, we will also produce 2 moles of H2O, and if we have 6 moles of H2, we will have 6 moles of H2O
4. Calculate how many grams will be equal to the number of moles that we found out, using the molar mass of H2O, 18g/mol
18g = 1 mol
x grams = 6 moles
x = 108 grams of H2O are produced from 12.1 grams of H2