Answer:
x = 4
Explanation:
You want the equation of the vertical line through (4, 6).
Undefined slope
Slope is the ratio of rise to run for a line. When the "run" is zero, that ratio becomes undefined. The "run" will be zero for any rise when the line is vertical.
Equation
The equation of such a line has the form ...
x = c . . . . . for some constant c
In order for this equation to be satisfied when (x, y) = (4, 6), the value of the constant must match the x-coordinate of the point.
The equation is x = 4.