Answer:
The slope is zero
Step-by-step explanation:
If the slope is positive, seeing the graph from left to right the line goes up, and if the slope is negative the line goes down, however if the slope is zero, it is a straight line that remains at the same point without going up or down.
Mathematically, we can define the slope as a <u>reason of change</u> between y and x, how much the line advances in y for a given change in x. Like this:

where
is the slope,
is the change in the y-axis, and
is the change in the x-axis.
In this case the line does not change position on the y-axis (for any value of x the line is at the same point in y), so<u> change in y is zero</u>: 
And therefore, the slope m:

Thus the slope is zero.