How do you Calculate the molecular mass or formula mass in (amu) for the following?

grams / mol

grams and amu are really similar. don't worry too much about the difference. (fyi it's 1 atomic mass unit = 1.66053886 × 10-24 grams)

for example: H2O it will be 18.02 grams / 1 mol

Alright what you want to do is find out all the atomic masses of the molecule. Note that either way, molecular or formula mass, will give you the same answer. After you find the masses of all the elements add them together. If you have more than one of the element, multiply the element's mass by the number next to it. So what you have for SO3 is...
S=32.1
O3=48 (16 x 3)
Note how I rounded. (I usually round to the nearest tenth)
When you do this add them together and you got your answer.
For the third problem, I you have to distribute the 2 at the end to both the P and O4. Do the same thing but multiply the two substances at the end.