We can see that b is a common factor. This leaves us with 8 and 12 for which the gcf is 4. So the gcf of 8b and 12b is 4b.
Note, if there are two different powers of the same variable in the two expressions, then the smaller power of b would appear in the answer (e.g. the gcf of 8b5 and 12b3 would be 4b3). If a variable appears in only one expression, it will not appear in the gcf.
For 36r2 and 18rq:
Note that the gcf of 36 and 18 is 18. Thus the gcd of 36r2 and 18rq is 18r.