The exponents for the variable terms must add to 8, which is the original outer exponent of the expression given (3a+4b)^8
For something like a^8, we don't have any other exponent so we don't have to worry about it. If you wanted, you can think of a^8 as a^8b^0 and then note how 8+0 = 8. So the rule still applies
For ab^7, we would write it as a^1b^7 and the rule works here as well (1+7 = 8).
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Something like a^2b^3 is a non-answer because the exponents add to 2+3 = 5 Same goes for ab^8 = a^1b^8 because we have the exponents add to 1+8 = 9 And for a^6b = a^6b^1 we have the sum of exponents being 6+1 = 7