Answer:
There are many options, like:
14/1 or 28/2...
Explanation:
First write the system:
(21 x 8) / 12
Then simplify the brackets:
(168) / 12
= 14
Then you can put 14 over 1 as an example of proportionality:
14/1 : (21 x 8) / 12
Or multiply it by 2, 3 etc.:
42/3 : (21 x 8) / 12.