Question

What is the slope of the line y= -3?

Answer 1

The slope is zero.
This is a perfectly horizontal line passing through `y=-3`. It is neither increasing nor decreasing so its slope must be zero.
You can evaluate the slope using:
Slope=`(Deltay)/(Deltax)=(-3+3)/(x_2-x_1)=0` this because choosing any two `x` values, `x_1` and `x_2`, the corresponding `y` is always `=-3`!