Answer: slope = 0
Step-by-step explanation:
Any horizontal line always has a slope of 0. The slope is rise/run. Horizontal lines don't have a rise or fall, so rise = 0. The run can be any nonzero value you want.
You can also use the slope formula
m = (y2-y1)/(x2-x1)
and you can pick any two points from that blue line.