For this question, you have to use dimensional analysis. It can be difficult to get used to at the beginning, but once you get used to it, these questions (and many others) will be a breeze!
The question is asking you to go from moles of C3H8 to grams of H2O. In order to do this, you will have to convert moles of C3H8 to moles of H2O and then moles of H2O to grams of H2O. **To go from one substance to another, you will always need to use moles and molar ratios.
The idea here is to make the units you don't want in your answer cancel out.
moles of C3H8 x (moles of H2O/ moles of C3H8) x (grams of H2O/ mole of H2O)
This setup allows you to cancel out moles of C3H8 and moles of H2O to be left with grams of H2O (what you are looking for). Depending on the question, you will need to change what's on top or on bottom of the fractions to get the result you need.
For the middle piece, you will use molar ratios from the reaction. For every 1 C3H8, you get 4 moles of water so you can plug those values into that part.
The last fraction has grams per mole of H2O. That's your molar mass. Use the periodic table to calculate the molar mass of water. It will be about 18 grams per 1 mole of H2O. Plug and chug!
I hope this helps. Let me know if you have any other questions about mole conversions and how to setup conversion problems
