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, we will do that by using the molar mass of O2, 32g/mol
32g = 1 mol
64g = x moles
x = 2 moles of O2 in 64 grams
3. Check the molar ratio between the two compounds, according to the reaction, the mole ratio is 1 O2 to 2 H2O, mole ratio 1:2, which means if we have 2 moles of O2, we will produce 4 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 = 4 moles of H2O
x = 72 grams of H2O are produced from 64 grams of O2