Use slope formula to find the slope for the following points :

Slope formula:  m  =  (y2 - y1) / (x2 - x1)

if you choose (x1, y1) = (-1, 4) then x1 = -1 and y1 = 4

                and (x2, y2) = (1, 4) then x2 = 1 and y2 = 4.

Placing these values into the slope formula:  m = (4 - 4) / (1 - -1)  =  0 / 2  =  0.

A horizontal line has a slope of zero.