Exactly as it is.
18/7 is in its simplest form.
7/18 is in its simplest form.
18/21 in simplest form is 6/7.
18 over 14 in the simplest form = 9/7 or 12/7
21/54 = 7/18
If you mean: 18/42 then it is 3/7 in its simplest form
14/18 = 7/9
18/42 = 3/7
3/7
2/7
7/18 in simplest form.
6/7