The answer would be: quantity x plus 6 over 6
quantity 3 x plus 18 over 18 can be translated into this:
(3x+18)/18
The expression can be simplified by dividing the (3x+18) with 3. Then it would look like this
(3x+18)/18
3(x+6)/18
(x+6)/6
You can describe (x+6)/6 as quantity x plus 6 over 6