what is y=-3 graphed?

What is y=-3 graphed?

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It's just a line parallel to the x-axis that passes through the y-intercept of 3 -  the point (0,3).

All points on its graph have a y coordinate =3, no matter what their x coordinates. Also, this line has a slope = 0.

You mean that all x-values have a y-coordinate of -3, right?

Yes that is right

Here is the graph. 

https://www.desmos.com/calculator/0fime4jc06

Sorry...didn't see the (-)....!!!