You can find the GCF of these two terms by finding the GCF of the individual parts. I'll start off with 8 and 12. You can find their GCF by making a list of their factors.
8: 1, 2, 4, 8
12: 1, 2, 3, 4, 6, 12
So, the first part of your GCF will be 4.
Next, find the GCF of the variables. Do this by listing the variables in each term.
Term 1: a , a , a , b , b
Term 2: a , b , b , b , b
They each have at least one a and two b's, so those will be the second parts of your GCF. Now, put them all together with the number first and then the variable with the higher power.
4b^2a