Final answer:
The equation y = 9x - 4 defines a linear function because it represents a straight line when graphed.
Step-by-step explanation:
The equation y = 9x - 4 defines a linear function because it represents a straight line when graphed. In a linear function, the exponent on the independent variable (x) is always 1, and there are no other variables or terms involving x raised to a power. For example, when x = 0, y = -4, and when x = 1, y = 5. These points lie on a straight line, indicating a linear relationship between x and y.