Let's plot that point and draw that vertical line through it: 



Let's look at some more points on that vertical line



What do you notice about all those points?  You notice that they
all have the same x-coordinate, which is -7.  So the equation is

"the line every point of which has x-coordinate = -7"

or writing that sentence much shorter, we have

x = -7

that is the equation of that vertical line.  You just write

what x (the x-coordinate) is always equal to on that line.

Edwin