You use parentheses only for grouping. So

    f a b (g x y)

is how you would write your example. In Elm, you could also use the function application operator to avoid writing the parentheses:

    f a b <| g x y -- EDIT: flipped the operator around

In Haskell, this would be done with $, which used to exist in Elm as well:

    f a b $ g x y

Once you get used to mentally parsing the $ as grouping the rest of the expression together--admittedly, it does take a little while--the last version is probably the most readable. At least that's what I've found.