Final answer:
The slope of the line that passes through the points (5, 6) and (5, 9) is undefined because the line is vertical, resulting in a 'run' of zero.
Step-by-step explanation:
The slope of a line measures the steepness and direction of the line, calculated as the 'rise' over the 'run,' which means the change in y divided by the change in x. For the given points (5, 6) and (5, 9), we can find the slope by using the formula:
Slope (m) = (y2 - y1) / (x2 - x1)
In this case, the slope is (9 - 6) / (5 - 5), which results in a division by zero. This indicates that the line is a vertical line, which means its slope is undefined. Vertical lines have an undefined slope because their 'run' is zero, and division by zero is not possible in mathematics.