Find the equation of the line that passes through 5,4 and 5,6

x = 5

Justification:

Realize that this two points are one directly (vertically) over the other.

This is they define a vertical line which is a line parallel to the y-axis.

That means that the line has a constant value of x, I mean the value of x is the same for every y.

The equations of this kind of lines is x = constant, and the constant, as you can notice, is the value of x which is 5 for both points.

So, the answer is x = 5.

As you see, this relation is not a function, because given the value of y you cannot know the value of x (there are infinite y-values that meet the condition x = 5).