The equation of a line that has an undefined slope is a vertical line. Vertical lines go straight up and down, and they are characterized by the fact that they do not change in the horizontal direction regardless of the vertical position.
Since the line must pass through the point (2, 1), we can determine the equation of this line. A vertical line takes the form of:
x = a
where a is the x-coordinate of every point on the line. For our specific line that passes through (2, 1), the equation becomes:
x = 2
This means that no matter what value y takes, x will always be equal to 2. So, the final equation of the line passing through the point (2, 1) with an undefined slope is simply x = 2.