Find the LCM of the fractions:
21=3*7; 6=2*3; 14=2*7; LCM=2*3*7=42.
Now express all the fractions in 42nds: 22/42, 35/42, 105/42, 27/42.
Just compare the numerators: 22, 35,105,27.
These are easy to put in order: 22, 27, 35, 105, so now you have the order of the original numbers:
11/21, 9/14, 5/6, 5/2.