Final answer:
The equation of the vertical line through the point (9, −1) is x = 9, representing a line with an undefined slope that crosses the x-axis at x=9.
Step-by-step explanation:
The equation of a vertical line through a given point has the same x-coordinate for all points on the line. Since a vertical line through the point (9, −1) will cross the x-axis at x=9 and has no slope, the equation of the line is x = 9. Vertical lines do not have a y-intercept and the slope is undefined because a vertical line goes straight up and down. The general equation for a vertical line is x = a, where a is the x-coordinate of the point through which the line passes.