Answer
slope = $0$
Work Step by Step
RECALL:
slope = $\dfrac{y_2-y_1}{x_2-x_1}$ where $(x_1, y_1)$ and $(x_2, y_2)$ are the coordinates of the two points on the line.
Use the points on the line $(0, 4)$ and $(2, 4)$ to obtain:
slope = $\dfrac{4-4}{2-0}=\dfrac{0}{2}=0$