(84/14)x(9/18) and 84/(14/9)/18 both equal 3, so they are both correct in terms of giving the right answer.
When evaluating them, you need to use the math order rules, so parentheses are always first. Next is multiplication, followed by division, addition, and subtraction. (I like using: Please Excuse My Dear Aunt Sally to remember them - Parentheses, Exponents, Multiplication, Division, Addition, Subtraction).
Whenever you divide a fraction, you can just invert it and multiply instead, which can make things easier to avoid double divides. Remember that a whole number is also equal to that number divided by 1.
84/(14/9)/18 = (84/1 X 9/14) X 1/18
You can now more easily cancel fraction terms.
84/1 X 9/14 X 1/18 = 6/1 X 9/1 X 1/18 = 9/3 = 3
(84/14)x(9/18) is much easier, so you just simplify
6/1 X 3/6 = 3