Question

How do I find the set of all points of y=-8 on a graph?

Answer 1

Any equation in x and y merely gives you a description of the relationship between the x and y coordinates of the points in a relation.
e.g.
x=y = 6 means that as long as the x and the y of a given point add up to 6, that point belongs.

So in your case it says that y = -8 , with no mention of x.
That means that we don't put any restriction on x and x can be anything we want it to be
as long as our y is -8, anything goes

so some points:,
(1,-8), (5,-8), (-3,-8), (3.56,-8), (√2,-8), (0,-8), ...

Answer 2

4th line should have been

x+y = 6 means that as long as the x and the y of a given point add up to 6, that point belongs.

but you probably guessed that anyway.

Answer 3

Can you please explain to me how you find the points, such as (1,-8) and (-3,-8)are mulitilying, adding, what exactly do I have to do to get this?

Answer 4

As I stated, since there is no x in your equation, x can be any value you want it to be, as long as the y value is -8.

so I just made up any x I could think of.
There is no mathematical operation involved at all, since all it said that y = -8 or
"the y value has to be -8"