4b because you distribute the 4 to get 8b - 4b so that equals 4b

pepper

you have to find a like term. the answer is 4b because when you do b x 2, that is doubling the variable of b. when you take b away from b+b, or b x 2, that is b.
the 4 is outside of the parentheses, so you multiply it, as the coefficient, by b. so this cannot be solved, only simplified to 4b