first you need to factor the problems by using a couple of different methods, but the easiest could be the factor tree:
for example with 4-6a
you would put 4 = 2•2 so 2 is it’s GCF
and 6= 3•2 so 3 is it’s GCF
however the question is asking for the combined GCF (meaning the whole problem)
so 4-6a’s GCF would be 2 considering it is the only greatest common factor between BOTH numbers
for 12a^2 - 8a you would use the same method:
12a^2= 3a•2a•2 and 3a is it’s GCF
8a = 2•2•2a and 2a is it’s GCF
but combined, the greatest factor BOTH numbers have in common is 2a, so 2a is the answer!
i hope i helped you :)!❤️