X = 4. The difference between the small and large sides on the left side is 1. (6-5=1).
You should take that difference and compare If to the size of the wheel left side, 6.
The difference should be equivalent for the other sides as well.
1:6 2:?
1:1 =2:12
2x+ 4 = 12
2x = 8
X=4