Your Answer is correct, because for every rate of change the x-value (-1) is decreasing, and the rate of change f(x)-value (-3) is also decreasing. This gives you a -1/-3 rate of change (slope). Two negatives make a positive therefore the is at a constant rate of change (slope) of 1/3.
Keep in mind unfortunately I am not 100% sure, but this is what I think. Hope this helps a little!
