Give a reason for your answer. Alright, so f needs to be both concave down and decreasing. So f concave down, concave down, that's going to be true if and only if the second derivative of f is less than zero, which is true if and only if f prime is decreasing. f prime is decreasing. So that's how we can tell when f is concave down. And I want to see what do I need to observe on f prime because that's the graph they give us. Alright, now for f to be decreasing, f is going to be decreasing if and only if, well you're decreasing when you have a negative slope, if your rate of change is negative.