Answer: y = 9
Reason:
The horizontal line equation has the template y = k, for some constant k.
In this case the answer is y = 9 to make sure it goes through (-4,9). All points on this horizontal line have the same y coordinate 9.
Other points on this horizontal line are: (0,9) and (5,9)
Put another way: The equation y = 9 means the y coordinate must be 9, but the x coordinate can be anything you want.
Extra info: The slope of any horizontal line is 0.