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. So that's true if and only if f prime of x is less than zero. So both true, both true when f prime of x is less than zero and decreasing. And let me write, and f prime is decreasing.