How would i calculate this and find the answer I want to wall paper a room 14'x12' and the ceiling is 9' high there is 2 3'x4' windows in it?

Multiply 14 times 12 times 9 ( = 1512 ) and that will give you the square footage of the wall surface.

You don't want to subtract anything for the Windows because the seams are never going to fall right at the edge of the window.

The square footage should be marked on the roll. But your ceiling height is important. If the roll has 40 linear feet on it, you're only going to be able to get 4 usable pieces off of it (4 x 9' = 36') with 4 feet of waste.

If the pattern in the wall paper needs to be matched, you'll need to order even more extra. If the pattern repeats frequently you'll only need to add an extra 10%. If the matches are a foot or more apart, add 20-30%. If you've never wall papered before, add 30-40%

...and check to see if the store will take back the extra.

Also, there will be "Batch Numbers" on every roll, make sure they match. The color between batches can vary.