I am not sure why kesavan7777 thinks the slope of the y axis is zero. The equation y=mx+a reduces to y=b for horizontal lines and to x=c for vertical lines.
The slope of the x axis is the limit of e/1 as e tends to zero.
The slope of the y axis is the limit of 1/(-e) as e tends to zero.
The product of these two slopes is -1 as e tends to zero.