Final answer:
The slope of the line represented by the equation y=12 is 0, because it is a horizontal line and there is no rise regardless of the change in x.
Step-by-step explanation:
To find the slope when y=12, we need to understand what slope represents in the context of a linear equation.
The equation y = mx + b, where m is the slope, shows that slope is defined as the rise divided by the run of a straight line. In the equation y=12, there is no x term, which indicates that this is a horizontal line.
For a horizontal line, the rise is 0 because there is no change in y-value no matter how much x changes. Consequently, the slope of the line y=12 is equal to 0.