I will assume the instructions for the problem was stated this way:

Find the equation of the line that passes through the two points
(-5,7) and (-5,3).

[If you post again, please state the instructions so we won't have to
guess. Thanks]

Let's plot those two points:



Now let's draw the line through those points.  I'll draw it in green:



Now let's look at some more points on that same green line.




Now take a look at all those points.  

Qestion:  What do all those points have in common?
Answer:   They all have -5 for their x-coordinate.

To make a statement to describe that green vertical line, we would say:

"All the points on that line have the x-coordinate = -5."

In algebra we shorten it to just this:   x = -5  

That's the equation of that green vertical line:

               x = -5

That's all there is to it!

Edwin