The domain where the function decreases is (4, 9] or 4 < x ≤ 9
In which domain the function is decreasing?
When we have a graph, a way to identify if the function is increasing or decreasing is to see if, when reading from left to right, we can see that the graph goes downwards.
Here we can see that we have a maximum at x = 4, and then the function goes down until x = 9
So the domain where the function decreases is (4, 9]