Question

Is it okay for slope to be zero?

Answer 1

Yes it is ok.
Slope tells you the INCLINATION of your line or how the value of `y` for a change in `x`.
Your line can be given in the form `y=ax+b` where `a` represent the SLOPE of your line.
If it is positive your line goes up!
If it is negative your line goes down!
If it is zero...neither up or down...so ot is a CONSTANT!!!!

Graphically you have:

Basically, when the slope is zero it means that you can change `x` buy the value of `y` does not change.
Graphically you get a line parallel to the `x` axis.