Question

How do you graph the line `y=3`?

Answer 1

Here is the graph of function `y=3`
graph{3+x*0 [-10, 10, -5, 5]}

Explanation:

Recall the definition of a graph of function `y=f(x)`.
Its graph is a set of all points `(x,y)` on the coordinate plane that satisfy the equation `y=f(x)`.

Obviously, all points `(x,y)` on the horizontal line crossing the `Y`-axis at coordinate `3` satisfy the equation `y=3`, regardless of its `X`-coordinate.

Any points `(x,y)` outside of this horizontal line has `Y`-coordinate not equal to `3` and, therefore, does not satisfy the equation `y=3`.

This proves that the described above horizontal line is the graph of function `y=3`.