18 because, 3 in the first round, 6 in the second round, 9 in the third round, 12 in the fourth round, 15 in the fifth round, and 18 on the sixth round.
Ask yourself this: "When is the graph completely horizontally flat?" The answer to this question is when x is between -4 and -2. This is where the graph is constant. All other intervals are either increasing or decreasing.
Saying "The graph is constant" basically means "constant rate of change", which is another way of saying that the graph isn't changing. Constant means it stays the same.
So this is why the answer is
Note how only choice A has both endpoints in the negative region, so it is a quick way to spot the answer.