Question

determine the domain on which the following function is decreasing.

Explanation:

Step1: Identify the vertex

The parabola's vertex is at $x=4$.

Step2: Find decreasing interval

For a downward-opening parabola, the function decreases when $x$ is greater than the vertex's $x$-value, and the domain ends at $x=9$ (open circle, so $x<9$).

Answer:

$(4, 9)$