The formula for the slope of a line based upon two coordinate points is
m = (y_2-y_1)/(x_2-x_1)
For the coordinate points (-3,4) and (0,4)
x_1 = -3
x_2 = 0
y_1 = 4
y_2 = 4
m = (4-4)/(0-(-3))
m = 0/3
This is a horizontal line with no rise, noDeltay.
The slope is
m = 0