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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 :)!❤️