You can put this solution on YOUR website! If y=-6 plot two points and draw the graph:
Points might be (0,-6) and (3,-6)
If you draw the line you will see it is parallel to the x-axis.
The slope of y=-6 is [-6--6]/3= 0/3=0
The same is true for the x-axis; the slope is zero.
Cheers,
Stan H