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.