I am not sure I understand your question, but are you asking how many grams of H2O are needed to produce 34.2 g of NaOH?
This equation is already balanced so to solve this problem we first need to know the molar mass of all the products and reactants:
Na2O: 2*(22.99) + 1*(16.00) = 61.98 g/mol
H2O: 2*(1.01) + 1*(16.00) = 18.02 g/mol
NaOH: 1*(22.99) + 1*(16.00) + 1*(1.01) = 40.00 g/mol
If we have 34.2 g of NaOH produced, then we can use stoichometry to find out how much H2O was consumed in the reaction:
(34.2 g NaOH)*(1 mol NaOH/40 g NaOH)*(1 mol H2O/ 2 mol NaOH)*(18.02 g H2O/1 mol H2O) = 7.704 grams of H2O consumed to produce 34.2 grams of NaOH.
Hope this helps!