The slope of a line can be calculated from any two points on that line.
slope=m=(y2-y1)/(x2-x1), where (x1,y1) and (x2,y2) are two points.
In this case:
m=(6-6)/(4--2)
m=0/6
m=0
Since the slope is zero, this is just a horizontal line, specifically the line:
y=6
