****Edit**** I can't see the whole problem, looks like there is more info but it's not showing so I don't know if the below is even applicable fair warning. All I see is .250 moles.
0.250 moles • 6.02x10^23 molecules = 1.51x10^23 Molecules of H2O
3 sig figs also.
Use Stoichiometry and Alvagadro's number (6.02x10^23) to solve this or any problem in the future.
Moles to Molecules or Atoms just multiply.
Grams to Molecules or atoms you must go grams to moles first. Do this my dividing the grams of the substance by it's molar mass and that will give you moles. Then, repeat like before and times by Alvagadro's number for the answer. That number is like what a "dozen" is to eggs. There are 12 eggs in a dozen. It is just a unit of measurment. There are 6.02x10^23 Atoms (eggs) in one Mole (Dozen). You should know this if your teacher gave this problem.