The graph for 'y = 3' is:

Three ordered pairs that satisfy the equation (point from the graph) are:

(-1, 3),(0, 3) and (1, 3)

The slope of a line is given by:

m = (y2 - y1)/(x2 - x1) where (x1, y1) and (x2, y2) are points from the graph.

If you take two points from the graph, the y values will always be the same.

Therefore, you will be dividing 0 by some other number thus the slope is '0':

m = (3 - 3)/(0 - (-1))

m = 0/1

m = 0