You want the equation of this green vertical line:



Let's look at some other points on that green line:



What's true about all those points?

Their x-coodinate is always 8, never anything but 8.

So the equation is simply the statement

x = 8

That's all there is to it!  Just x = 8 because that
is true for every point on that vertical green line.

--------------------------------------------------

You want the equation of this green horizontal line:



Let's look at some other points on that green line:



What's true about all those points?

Their y-coodinate is always 3, never anything but 3.

So the equation is simply the statement

y = 3

That's all there is to it!  Just y = 3 because that
is true for every point on that vertical green line.

--------------------------------------------------

You do the other one.  The equations of vertical and
horizontal lines have the simplest equations of all.
Vertical lines always have the equation "x = something".
Horizontal lines always have the equation "y = something".

Edwin