24.Hint

It's good to specify some properties about these functions up front. For example, do you expect these expressions to be true?

xs.append(ys).startsWith(xs)
xs.startsWith(Nil)
xs.append(ys).append(zs).hasSubsequence(ys)
xs.hasSubsequence(Nil)

You will find that if the answer to any one of these is "yes", then that implies something about the answer to the rest of them.