#6 Line has negative slope therefore minimum value occurs at right endpoint. So solve f(2a) = 8 for a ....
#7 Solve f'(a) = 1 for a. Then b = f(a) = .....
#8 I don't like any of the answers or the question for that matter. But they probably want the answer corresponding to .....
#9 Evaluate .
#10 Option (d) is wrong because it fails y(0) = 0. So test two of (a), (b) or (c). Either one of the two is correct, or both are wrong. Either way, you find the correct option.